Retying Your Belt
Tie your belt loosely at the start of the class.
Remove your untied belt, and carefully reposition and retie it.
Played well, it appears that you are the hapless victim of fate because the
knot of your belt "just came undone."
If you are wearing a hakama (the pleated skirts worn in some traditional Japanese arts) you can attempt the advanced variation of this technique that involves tying an unneccessarily precise and fiddly bow using the tapes at the front.
How to tie your belt was almost certainly the first thing you were ever shown when you started training, so performing this technique frequently annoys or enrages all senior partners. Often they will forcibly tie your belt very firmly thereby rendering this tactic void for the remainder of the lesson.