when you post a comment to someone's status update, and after refreshing you realize someone else made an almost identical comment at the same time.
JOHN DOE (status update): omg, can't wait to see Burlesque!
jane doe (comment, updated 8 seconds ago): it's showgirls 2!!!
jimmy doe (comment, updated 10 seconds ago): it's like the showgirls of our generation!
jane doe: omg, we just had a comment collision!