When someone says "nevermind" or "it doesn't matter", leaving you to wallow in your ignorance, forever wondering what it was they decided not to say.
Like when someone tells you NOT to think of an elephant, you think of an elephant.
-Oh my god, you know what's the most amazing thing ever?
-Oh, nevermind. It doesn't matter.
-Dude, elephant syndrome here! You're gonna kill me.