This trope refers specifically to instances of characters turning into this week's Big Bad (excluding say, were-anythings, and characters who turn evil for extended periods of time). Supporting main characters are particularly prone to this.
Eg.: Live Action TV/ Buffy the Vampire Slayer Xander and some other students in The Pack, possessed by hyenas (seriously). Joyce and other mothers in Witch, possessed by demon.
Should this trope be limited to major players only? It's a broad net otherwise.
- Superman's friend Jimmy Olsen had a comic back in the Silver Age that had him become the monster of the week on several occasions.
- Champions. An early supplement mentioned the "radiation accident" - an event that could cause a normal person to gain super powers. One possible plot was for a non-powered NPC or low-powered PC to have a radiation accident, get powered up to incredibly high levels and fight the rest of the PCs.