Specifically Lego or BIONICLE pieces you personally regret painting over.
For example, when I was younger during the old BioTube days, I decided in my infinite wisdom to make my own version of EDICTARTS mostly custom painted MOC, Toa Edictarts. There’s still remnants of gold paint in Jaller Mahri’s mask to this day…
Also the myriad of lego clone troopers who never survived the onslaught of colored sharpies.
I remember I named the purple one Sergeant Haze and I think the blue one was just “The Commander”. They’re both very special to me because they were my first two clone/stormtroopers I ever got; the clone in particular being from the 2009 Clone Walker polybag.