Having been a GM for almost two decades, this has somewhat ruined being a player in p&p rpgs for me.

I joined a new long-term fantasy campaign. Me and another forever GM plus 3 rookie players.

The first adventure starts and the group gets hired by some guy to retrieve an item from a dungeon while insisting to come along with us.

Me and the other forever GM looked at each other: That dude is going to betray us and become the BBEG of this campaign!

The rookie players looked puzzled.

The GM sat there pale and grey.

Of course we were right. Dude grabbed the item, teleported away and started his reign of terror. But we just chuckled and went along with it, much to the GM's relief.