The thought crossed my mind while replying to another post. For myself I built what I believe was a Revell 1/32nd Spitfire way back in the 70s that allowed for multiple versions. As a result I ended up with an extra cowl scoop and a few other parts that at last recollection are sitting in a parts box somewhere around here - among quite a few other boxes. Molded in shit brown so they stand out among everything else and always catch my eye and take me back a ways. A close second would be tires and wheels from car kits. Rare is the old modeler without a healthy supply of Cragar mags and Mickey Ts. Rare is the old modeller who hasn't spent far too much time sorting thru them trying to find complete sets of four to no avail!
Let's keep it somewhat honest - parts from kits you actually built and tossed into the scrap box or kits you scrapped out at some point. I've got older kits and scraps I bought off ebay and elsewhere - what I'm angling for are the kits you bought, sat down and built, then time took its toll or you had leftovers after the build. And for some reason known only to you and other inmates at the Testors Tube Glue & 15-Cent Bottle Paint Modeler's Home - you hang on to them.
WmB