Even HE cringes at the mere mention of it. I have toyed with the idea of doing it as well but, on that occasion, I went home and took a nap until the feeling went away. 
Do you mean that you would be makeing up the same print that you are running? SOund a little scary to me.
Just remember to be careful and run the make up table SLOWLY. On certain kinds of platters (Speco comes to mind) you can blow the main fuse when you "crank" the speed control while the show is under way.
Another thing to remember when using Speco: The bottom of the decks on the Speco have metal battens on the bottom which will act like a giant fan when the deck gets up a good "head of steam". If you are running the show between the top and bottom platter, for instance, and you are breaking a print from the middle, the "fan effect" will cause the film on the bottom to get tangled; thus risking a brain wrap.
The solution is to make sure you build or break from the bottom deck or else run the break down table SLOWLY.
There are certain times in a large theatre when you pretty much have to build and break a print while another is running on the same platter. You just have to learn the peculiarities of your system and make sure you know how the proper "work arounds."
We normally try to breakdown a print on the middle level and make up on either top or bottom.
For the Tower system here (as Pete said), I can very easily make-up (always checked on 6,000’ spools first) or break down into 2,000' on the other side during a show.
If you are talking about making up or breaking down a different print on the same platter system while another is playing, that's no problem. I always use the bottom deck in either case because of those platter support arms that act like a fan, as Randy said. It's not just with Speco.
I just didn't mention it because if you do put a larger one in, you can potentially cause other problems. For instance, if you accidentally press two green buttons at the same time and both payout controls, for some reason, get moved into the "run" position at the same time, the current surge can casue things to start frying. You could simply require changing out the payout control card(s) or you could end up replacing one of those pushbutton switch assemblies.
If you put a 3 amp fuse in there, as recommended, you'll simply pop the fuse. If you put a 5 amp fuse in there, there is potential for trouble.
If / when I put larger fuses in the platter, I make sure to warn people about not pushing two buttons of the same color at any time. (Except for prescribed emergency procedures.)
Randy
interesting about the power surging the boards...tells me that the three amp fuse used in the payout control either is not three amps or needs to be smaller to prevent this from happening....this fuse should blow if anything above three amps is introduced.....I was told that putting a five amp fuse in the main inline power box would be fine so long as the fuses that protect the payout and the take up were not changed to a bigger fuse.
On loading a film that is currently running, this is incredibly stupid. I'll admit I have done this before and why I did is beyond me. Nowadays I would much rather go into the auditorium and tell all of the customers that the print arrived late and that they can get their refunds by calling 1-800-99-FILMS. If you have to load a film that is running because someone has a gun to your head, then you must remove the belt in the MUT and have the majority of the film already on the platter (say the first 4 reels of a 7 reeler). Still though, no performance is worth the incredible risk to the print from doing this. The slightest bounce from the reel will throw your print off of the platter.
There were many times that they delivered prints late. Generally speaking, we received our prints between 12:00 and 3:00 on Thurs. If they had not delivered by 3:00, then our home office as well as the distributor was notified.
In my opinion, if the projectionist does not receive the print by the early afternoon proceeding opening day, then it is his/her responsibility to notify theater management, the home office (if your company has one) as well as the distributor and shipper. There should really be no valid reason (unless a major aw shit occours) for films not to be prepped and ready to go on opening day.
In any case, trying to build a print as it is running is unacceptable and is looking for trouble!
If a major aw shit occours where the print is not delivered on time, maybe the distributors and shippers should be charged for the lost revenue?
Another thing I have seen is that people have put Slow-blow fuses in, thinking that would help. This is another big NO-NO! Luckily, I caught that one in time and I reamed out the guy who did it, then made him go around and put all the right fuses back in.
The most common problems I have seen from putting the wrong fuses in are blown payout control cards and fried contacts on the switch banks. The second most common thing is a fried variac. Replacing variacs and payout cards is pretty academic. It's just the cost and inconvenience (and lost shows) that causes you the most heartache. However, changing out those switches is a royal pain in the arse! I have had to do it about 1/2 dozen times over the years. Unsoldering the wires and reconnecting them is just about the worst job you'll have to do when it comes to repairing Specos. You only have a few inches of wire to work with so you don't have much room for error when it comes to cutting and stripping wires. Not only that, you don't have much room to manuver, even when you take the whole switch bank apart.
The fact that Speco has changed the design of the switches doesn't help much. You have to figire out where the new connections are and if you don't get it right, you'll cause more trouble. I had to undo somebody else's handywork in that area once and it took me a whole day to first figure out what was screwed up and then to redo it again correctly. IF you have one of the newer Speco designs the switch change is rather painless because they have made it so that they just plug in. The problems come when you have to "retrofit" a new switch into the old wiring scheme.
I have no problems with people putting a 5 amp fuse into the bottom of the tree as long as they remember to never press two buttons of the same color at the same time. I know accidents happen sometimes but with some people, it's like talking to a brick wall. (The instruction manual plainly states that you should never do that, even with a 3 amp fuse.)
If you can't drum in into people's thick skulls how to work the equipment properly I just think it's better to leave a 3 amp fuse in there, like it was originally, and leave well enough alone. The only problem is that you either have to drum it into their skulls about the buttons or you have to drum it into people's skulls about not running the make up tables at top speed. It's a catch-22 situation. That's why I didn't mention it in the first place.
At least, with the Christie MUTs, I can "handicap" them by lowering the maximum speed control so that they don't run too fast. I have taken to doing that at selected locations because they were scratching film when they were making up the movie for the first time. In places where this has happened, handicapping the tables has minimized the problem.
As to simultaneously building and showing a print, I suppose it's theoretically possible. OK, so Brad has done it but he's the Master. Even HE cringes at the mere mention of it. I have toyed with the idea of doing it as well but, on that occasion, I went home and took a nap until the feeling went away.
It did work, but I don't think I'll ever do it again. That was just stupid. The safety of the print is not worth the few extra minutes saved in loading it properly.
German Marin from Venezuela.
A long time a ago there once lived...
Thursday night, ready to screen. Try to get the (Christie) center ring out and guess what. The belt got jammed on the mut and created an incredible amount of back tension that the ring was actually compressed closed.
So, down onto 6K's it went. It wasn't until about the last trailer was off did the ring loosen up. So I thought, hmmmmmm, spend another 30 minutes or so spinning the print back on, or .... screen directly off the 6K's and let it build back up on the platter.
Ok, so set the MUT between the platter and the projector to avoid any twisting, go from the bottom mut roller to the top platter roller. From the bottom projector roller through the feet of the mut, carefull of the cross bar of the MUT, and continue to thread the platter regularly.
When it gets close to the end of the reel, changeover time. Worked incredibly well with no damage. I wouldn't do this as habit, but it saved time on a night that was starting late to begin with.
... and they lived happily ever after.
IMHO, there is a 100% chance you will cause dirt and scratches, and the risk of severe damage is very high.
What I ended up doing was breaking down the print to the end (beginning 
) of the wrong reel, splicing it back together the right way and running the rest of the show off a 6,000 foot reel by placing the make-up table between the platter and the projector.
It's something that never should have happened but, none the less, it did. In a situation like that you have to do whatever is necessary to let the show "go on". I would never do something like that unless it was absolutely necessary. Thus, I would classify any method of running a showl like this, other than the prescribed methods, as a last minute, "save-the-show" technique.
In such a situation, I might add that it is well worth taking an extra five minutes to stand back and look at the problem to come up with a logical course of action, rather than responding with some half-assed solution that might cause further interruptions down the road. Customers don't like interruptions like this but when presented with some problem like this they understand that "shit happens". They have a certain tolerance for problems like this and might very well give you a "Mulligan" BUT they are likely to become VERY pissed off if something else happens. It, therefore, becomes necessary to make the "fix" on the first try.
That having been said, it's MUCH better to let people wait for two extra minutes while you get your shit together than to make them wait for a half hour while you run around like a chicken with its head cut off, making one mistake after another.