What part(s) have you let go, only to wish you had kept them not long after?

So I recently sold a flywheel because that i picked up in a free load of parts i got from one of the guys who work for me because I just never expected to come across a 4spd at the right price (free) to justify the possibly of converting my 68 B'cuda ragtop.

As luck would have it, a friend of mine is cleaning out and is giving me a 4spd from the Duster he had wrecked back in the 80s. Bell housing to output shaft.

Poop!