A F5 moment is computer gamer talk for a Kodak moment, a moment when a picture explains what the hell just happened better than any words could.
F5 is the key used to take a picture in some Steam games.
Last night on L4D2 Archie made a smiley face on the wall with bullets, it was a real F5 moment.
When Payne pulled off the chainsaw hack and painted the saferoom red with zombie blood it turned int a real F5 moment.