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

I gave a brand new NOS 1945 Carb for a slant six away to someone a few years ago.
At the time I thought I was helping someone out.
Turns out that I was, the new carb helped him to sell the car only for the new owner to pull the slant out and drop a 360 in it.
I asked for the carb back and the owner said he tossed it.