"Errare humanum est, sed in errare perseverare diabolicum" (Seneca, 62 A.D.)

To err is human, to stay in error diabolical.

Essentially, there are two type of errors:

the "bad" errors (often happen only once)

the "innocent" (unpleasant but useful minor misstep) 

Turn to Your neighbour, discuss the question "what was the worst error (e.g. the biggest fuckup) You ever did/caused in Your life". After 5 minutes, each couple will be asked to present at least one such error to the plenum.