One of my first GM experiences I used an NPC who was a former PC of mine: Sir Tamen, a haughty noble, stern, severe even, but whose heart was in the right place. I meant for Tamen to be a patron of the party, directing them toward their next mission, aiding them when necessary, nudging them if needed.
Instead, Tamen stole the show. He gave out the mission, then, as if taking on a life of his own, lead the party on the mission, charged through the enemies, and defeated the big bad. And he was a stern, severe, prick. The players hated Tamen, and I really couldn’t see why.
In response to my horrible DMing, the party betrayed and attacked Tamen whilst unarmored and without spells (he was a paladin, back when that meant something). Their assault was well-planned, and well-executed, and Tamen was all but dead in one round. Or he should have been.
Instead of allowing Tamen to be defeated so easily, I made up some bullshit about some at-will spells attached to his sword, one of them being healing. The next round it was a holy sword that could summon his armor. At the table the players cried foul, but I played the, “I’m the GM.” card and blatantly lied, saying the sword always had those powers.
It’s hard to admit, even now, even knowing that I was young and inexperienced and every GM makes those mistakes early on. Tamen shall remain retired, a reminder of what not to do.