three occasions that I can remember right now...
- Pot smokers in the room next door.
- checked into a suite (complementary upgrade) and noticed that my window was right above the hotel's HVAC system. I had a room facing the same direction at that hotel on a previous occasion and knew how loud it was going to be, so went right back downstairs and asked for a new room (and got a regular room which I was very happy with).
- fleas