I forgot you Exist
This phrase is used by someone who’s so full of themselves that they think the other person isn’t even worth remembering, and they’re just trying to avoid being called out for being an idiot.
'I forgot you exist?' You're still here, so why are you still talking?
You forgot I exist? I guess you're still an idiot.
I forgot you exist? You’re still breathing, so you're not dead yet.