Red Flags for DMs?

[Socksy]

i can't tell you how many times in the last 10 years i had to learn the rules of whatever new class/etc. A player wanted to play.

You know why i have to know the rules of their classes, and can't just go off of their say-so? Because when i do i constantly have this interaction in the middle of a session:

Player: My character activates such-and-such ability.
Me: Which does what?
Player: It allows me to do [some ridiculously overpowered thing].
Me: Wait, it does what?!

*looks up ability*
*reads it with knowledge of what the game terms actually mean*

me: Um, no. What that ability actually allows you to do is this [other, non-game-breaking thing].

I take a deep breath, start drowning, and Iron Heart Surge the lake into nonexistence!