Here are the top 10 I have seen as both a passenger and airline employee, not in any particular order:
1) Live bat discovered in the cabin in flight, was trapped in the over head bin halfway from YEG-SFO, after landing 2 rampers and animal control had to chase it around and catch it in a potato sack before the next flight could board.
2) Old man has pooped in his duffle bag, tried to jam it in overhead on the Brasilia. It's everywhere...everywhere.
3) There is a crack on one of the emergency exit placards(nope, wasn't gone, just a crack in it) and the pilot refused the airplane...so of course we had to do an aircraft change that took 3 hours and half the passengers misconnected in YYC and were stranded.(I want to punch that pilot in the face everytime I see him).
4) Passenger walks from row 3 all the way to the back of a CRJ-700 to use the lav, doesn't quite make it, then walks back to his seat leaving a trail of poop behind him. Gets off the plane and is angry he can't have his checked bag in the jetway and calls me a choice word. Then as if I didn't love him enough, I found out he did this. Then had to clean it.
5) Flight attendant refuses and airplane because the door won't close right. Both pilots and mechanic insist it is closed right, she refuses. They do a plane change, she refuses the next one too. Again, everyone else thinks the door is secured. She refuses to work the flight. Flight cancels after a 2 hour delay. She was fired for something trivial about a week later.(No sympathy here, those poor people were stranded for 2 days in San Francisco and I had to rebook them alone until almost 3AM, at which time they didn't even have hotel shuttles for her pax).
6) After the doors are closed and the parking brake is released, CA suddenly realizes her DEN airport diagram is expired(Isn't that what pre-flights are for?). It takes 40 minutes for UA to secure her a new one, I then have 6 minutes to run from terminal B to A to catch a Frontier flight. God forbid they built a new runway or changed all the frequencies overnight.
7) The autopilot is out on an airplane that is to do a 35 minute flight and then overnight. F/O says he doesn't care(it is his leg), CA refuses the airplane. They do a swap, lav is inop, he refuses that airplane. Flight cancels.(Another pilot I wish to bump into in an alleyway downtown one day).
8) Cargo door light is out on an airplane. MX tells the pilots they will just have to add visually checking the cargo door once the rampers close it to the preflight. CA says "No, that's extra work". MX and I laugh thinking he's kidding. He refuses the airplane. 2 hour delay for a swap. Same guy from number 7.
9) Flight is pushed back, during pushback the ramper advises the CA that there is one extra bag, he has changed his copy of the load report and asks the CA to do the same. CA agrees. Halfway to the runway, the CA decides he wants the rampers signature next to the change(this is NOT company policy or anything like that), right after an arrival rush, they hold in the alleyway for 1+ hours waiting for a gate. Pull into a gate, ramper changes it. They wait 20 minutes to call for push doing god knows what. They time out. Flight canceled.
10) Airplane's boots won't inflate, it's overcast, they can't deffer it. MX installs new part, takes one hour, part is faulty. New Part is flown in from Fresno, 2.5 hours later it has arrived and is installed, its also a bad part. A flight from SLC is about to land and coincidentally has several COMAT shipments onboard for our MX department, including the needed part. It arrives, they install the new part, its about another hour and 20 minutes at this point. THAT part is faulty too. Flight cancels. An hour later they install a second one from SLC on the plane which is now overnighting, that one works. Good times. Most of the passengers wanted a refund, refusing to get on one of our airplanes the next day. I don't blame them.