Here are the two i can remember:
1. Just about to land in Alicante for a weekend trip (student days!). Look for my wallet. Not in my bag, not in my jacket, not in my trouser pocket. Wallet had slipped out of my pocket whilst i was sitting at the departure gate. Very luckily it had been handed in to the airport staff. Incredibly it still had my 100 euros in it and £50. This on top of a very kindly woman who saw my distress and gifted me 20 euros so I could get the bus to my girlfriends house and get myself something to eat/drink.
2. At the gate, hand over my boarding pass and then head on down the air bridge. 2 minutes later shamefacedly make my way back to the gate. Guess who had forgotten their boarding pass and been denied entry to the plane?