It occurs when, in a public situation, someone hears an interesting word and doesn't want to look like an idiot and ask what it means. At that time, thinking to themselves "I'll look it up on Urbandictionary later!" then, upon reaching an internet source, you experience a total blackout and cannot remember the word you were going to look up.
Me: NICE dude *thinks* (what the fuck is that?)
*later on that evening*
Me: okayyyy *urbandictionary* FFFFFFFUUUUUUU WHAT WAS THAT WORD!?!?!? D:
*Suffers from Urban Amnesia