Hahaha, a well funny thread.
Probably when I was drunk and stayed overnight at a friends house. I couldn't find the toilet so peed into her kitchen sink. Disgusting and disgraceful behaviour, I'm not proud of it at all.
The most morally disgusting thing I've ever done was having sex with two different girls in the same day. Whilst it was morally disgusting, I enjoyed it if I'm being honest.