If entered in mIRC, a command that disconnects the user from all their channels. Use as follows:

<bob> Hey, have you seen this amazing new channel! It's full of GUNS 3D ALIENS FIRE SEX WAREZ 1337 GUNS!
<bob> just type /join #3,0
*** Quits: QUAKE_GOD
*** Quits: EVIL-LEET
*** Quits: *d4*m45t3r*
*** Quits: Zero_cool776
*** Quits: neo24045
*** Quits: DEMONKING
<bob> Fly, fools! FLY!

The reason it makes a user part all channels is because, once upon a time, IRC used numbers instead of channel names (e.g., #stuff).

Since the comma works as a delimiter, the person really is joining the channel #3, then joining 0. Back when the channels were numbers, 0 was the equivalent of not being on any channels. So, when you join 0, you part all.

I first saw this used as someone saying to "Join #2,000" so, #3,0 is not the only way this can be done.