I think it is when someones connection is fast the other person's is slower than the other persons. When the action gets hot, one lags while the other maintains a solid connection thus the two aren't in perfect sync. Its sort of like doing live video on 56k. You know how things don't "stream" on 56k. Well that is sort of like online gaming. For things to run smoothly you need to have a good connection so no one gets out of Sync. Did this person have a good connection? This has happened to me before.

FROST is right about what it is normaly, but I think the guy zenobia is playing is pulling out early
What I mean is, he is pulling out his connection before it registers as a loss for him, it's just another form of cheating, rather pointless really...

Yes we were both at 4 star/ full ping. When the connection is bad it specifically says "Weak connection lag" which is a different message than connection sync error...very strange. Also it doesn't happen when this guy "Hammer" beats me only when I beat him....

He/she is pulling out early for sure. I had this happen on both PS2 and Xbox. Its a cheap way not to take a loss.
The funniest one I ever saw, I don't recall the name, but they had 600+ wins w/o a loss and numerous disconnects.
I don't particularty like it especially now that I have more losses than wins mainly because I would not cheap someone like that. I look at it this way I earned my record, it is mine. If someone else is better than me or beats me they deserve the win.

Sync failure is when your hitting the character after the match too much it does it all the time, possibly a drop. When your about to do your fatal and it says connection not responding that 100% is a drop...

They are just pulling out on you. If they are already dead and your game freezes or it says connection sync error its most likely because they are pulling out the cord from the network adaptor to avoid a loss on their record. Mh-Leader does it all the time. Everytime ive played him it always happens and its no coincidence.