For example, a tattoo can semi-permanently mark two people, indicating their relationship. Human relationships and individual behavior are unpredictable compared to the permanence of tattoos. Of course there's laser, but it's neither cheap nor perfect.
What would be an appropriate way to warn someone that the results of a decision can far outlast the foresight of the decision maker, that some things can't be easily undone and one might want to think about this a while longer?
"Bonus points" for a reference to decisions made in youth.