A guerrilla chat is when you get on Facebook or other service with instant messaging, find a friend online, send them a message, and then log out before they can respond. Sometimes done unintentionally, but almost always annoying to the person that finds your attention span was too short to wait for their response.
Hey, I think I just got hit by some guerrilla chat. I stepped away from the computer for 30 seconds, came back to find Kristy asking how was my day. Then my response was rejected, saying "Kristy is now offline."