The one for me must be the time I was trying to set up the sound in a Belgium theatre. We put the notice on the door "DO NOT ENTER" in English & Belgium.
As we EQ'ed the room we noticed a spike.
What can it be, air ducts or our RTA up the spout.
On closer inspection we found the fault. It was German video installers drilling holes in the roof to fit there projector.
They insisted that they did not understand the notice & they were not going to move as the cinema owner wanted the video projector
inplace by the end of the day to show adverts.
as we all know the most important thing in a cimema is to be able show adverts.
Booths / theatres that aren't finished when they are supposed to be.
Screens that arent't the right size so your picture doesn't fit. (The architect changed something and didn't find it necessary to inform you is the usual cause)
Electricians etc, 1. taking FOREVER to do anything 2. Always being one item short to complete something 3. Making as much mess as is possible 4. Leaving rubbish, tools EVERYWHERE.
Installing in a building with no heating working yet, in winter.
And yup, people making all sorts of noise whilst you are trying to EQ
A architect was asked to design a 4 screen cinema in a old office building.
He went to a local cinema to take some measurements.
He could not get in to the projection room so he went in to the auditorium to take the measurements. The porthole from the auditorium was 6 ft from floor level.
When we arrived the first problem we came across was the Projection room floor was at the same level as the back of the auditorium & the porthole was 6 ft off the ground. So the builders had to build a stage to put the projector on.
The second problem was the screen, after we had been given the room dimensions the architect added two air ducts ether side of the screen. We had to cut 2 Ft out of the screen frame & mix & match the Lenses. Also we had to rap the screen round the frame. This was not on the one screen it was all of them.
He came to the opening day & was as pleased as punch about the cinema he had designed.
I myself would have been pleased to punch him & came very close.
If I made as many mistakes on the average job as the architect does, I doubt greatly I'd be doing any more jobs!
"Contractors that think platter are lazy susan tool holders"
- They also think they are in a Chinese restaurant by placing their food and drink there too.
"Ladders placed on platters"
- Ever wonder why the platter isn't level and prints get thrown? 
"Unfinished floors and drywall"
- So that's where all that white powder comes from? Try to clean the print with a dry media cleaner and enjoy the scratches. 
"Air ducts in General. Mechanical contactors think they can put ducts where ever they please despite what is on the drawings"
- Letting water (condensate) drip from the ducts onto the plattered film underneath, as the film flaps in the breeze.
"Electricians finishing off the wiring to the popcorn machine before starting the booth"
- They need something to snack on while they work. 
------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion
Well, the electrician did. But there was a terrible error. The consoles were not "spotted" yet. The electrician never bothered to ask if the consoles were supposed to be in position.
They were displaced at all weird angles, and sitting where they were plopped and uncrated from shipment.
After the wiring was completed, the raceway work did not allow the consoles to be spotted properly.
That has to be a real "Duhhhh" to say the least. 
Has anyone ever heard of an equal or worse disaster in a new booth?
P.S. I visited that booth several months ago, and it actually was a nice booth. But why EVERY screen had a DTS, SDDS, and a CP-500 hard wired into it is beyond me. I guess the guy who built it had a lot of money to waste.
As a prop-man friend from IA Local 720 used to say: "I was in this business 20 years before I realized 'f**king electrician' was two words."
Paul
Former stage electrician
"How tall's a stagehand?"
"Don't know. Never saw one standing up."
Suited up, initially anyways, in a hard hat (which keeps getting knocked off every time you get in the unpleasant bending positions one is forced to assume during an installation), coupled with work-boots the weight of rocks, that put a resonance into your walk so you almost crash into walls the first couple of days or so, makes for an quite an experience.
Having done quite a few of these over the years, I would have to put (in no particular order) my pet peeves as follows;
Having the floor-surfacing not yet finished so ya gotta play chess with the equipment while they finish applying tile - if you're lucky enough to get tile.
Electricians (and other trades) who don't understand that we can't really get started until they finish their work, and electricians who don't correctly number the surround lines, if at all.
Constant dealing with various re-interpertations of the plans as to who is responsible for what work - eg. connecting speakers (we end up correcting it anyways - especially in bi-amped installations).
Trying to keep the consoles and sound-racks covered with plastic. (and, as Gord says, keep platters from being used to hold material to be hack-sawed).
Politics and competing installations resulting in your not getting your screens up till less than 24 hours before opening to the public, resulting in light-speed sound equalizations and aperture filing (which you know you're gonna wince at, on subsequent visits to the theatre).
Major last minute changes in booth-design or installation philosophy. ("What do you mean we open with THREE 70-MM features tommorrow?")
But I suppose the biggest gripe of all are the three questions asked time and time, again on every installation;
(1) "Exactly how do these things work?"
(2) "When ya gonna start the dirty movies?"
(3) "How much are these cameras worth?"
The rep from the slide company comes in looking for the "projector guys" and says "I need to get these set up and tested today". To which you respond "Well, you better tell the electricians" pointing at the guys in the ceiling trying to remember which speaker wires comes from where...meanwhile, there isn't even a electrical trough mounted on the wall yet.
Installing "like new" equipment that either smokes or does nothing at all when power is applied.
Everthing Rick L. said including the quotes...and in addition:
"My home theata's betta dan dis."
I'm sure it is buddy.
Sounds very much like you are describing my cinema, as i remember you and your team coming in to do our install. Yes the architect did an amazingly bad job the list could go on and on as you well know. For your information we still have no working masking or curtains in number one. I think the money ran out on the building refurbishment because i have spent the last the last 6 years working with sub-standard projection equipment (CFS platters, anyone?)
Also, the architect in question often comes in, where we take delight in imagining his film experience when the bingo from downstairs can be heard quite clearly during quiet moments in the film!
Best regards to all at Jack Roe
People who hack mid range speakers out of Sonics cabinets with axes .
Having to carry 60lb boxes up 105 stairs because the lift is out of use for two days to have a picture screwed to the inside .
Contractors who use underseat plenums as rubbish dumps and toilets.
Carpet fitters who cut through the induction loop tape for the third time in two weeks.
People who turn off the site power when you are 40' up a screen tower.
Finding out that what you thought was a straight continuous 55' conduit run through the roof space actually has a 20' section missing in the middle just where you can't get to it and of course there's no draw wire.
Electricians who pull your speaker cable runs for you and leave either:
1) 3" tails sticking out of the wall at either end or
2) 40 feet at either end all tangled up in heap and covered in rubble and guess what none of the runs are identified!