Ordinarily a towel wouldn't be that weird, but I had stayed in a youth hostel the night before my flight and the towel was still wet from using it that morning. I had it out so it would dry off, and for some reason one of the flight attendants thought it was funny.

Another time I was coming back from New Orleans on a Southwest flight. As we were taking off some people across the aisle began furiously ringing the call button. Apparently someone was bringing back poorly packaged fresh seafood; the bag was leaking and nasty water was dripping over the unlucky people sitting below that overhead bin. We never got to see who sheepishly claimed responsibility.