This can be anything, plopping a squat, nose picking, toenail biting, just all the TMI's you wished you've never seen!
I've walked in on my roommates son:
Watching porn in the livingroom on his Mom's computer [mind you I had my FIVE year old brother with me.]
Peel dead skin off his foot, eat it, spit it back out and exclaim "this tastes funny."
Moon me, showing off that he stuck a lighter up his ass.
My five year old brother:
I went to his field day and he was picking his nose, several times in front of everyone.
[This is funny but still, it was me] Sit on my head and fart.
Biting his toe and fingernails.
[He only did this once, when my ODC, ultra-shy brother was in the car, when we were younger] He used to child lock all the doors and turn the AC off when he farted and burped, and sang oldies.
Using his nose hair trimmer.