Things that seem like a good idea at the time often aren't

So there's that category of action which, at the time of conception or in the theoretical stage, seem like a valid and justifiable thing to do, but upon entering the practical stage, are just really, really bad ideas.

Things like:
Seeing how fast you can chug a gallon of milk.
Testing the reflexes of a pit bull.
Fishing on a bed of ice during a thaw.
Dying all your clothes the same shade of blue so they always match.

Newest on the list:
Doing a 2000 piece puzzle of Van Gogh's Starry Night. Yes, I finished it, but damn if I wasn't really sick of the thing by the end...