Token in form does not match token in server

This one's been bugging me for a while.  I have one user, maybe more, that gets the above error message every FIRST time he tries to login.  His second attempt always works.

I've gotten a somewhat similar message if I leave the main page up without logging in for a while, but that one makes more sense.  It tells me the form has expired.  This one doesn't mention expiration, just a mismatch.  I don't know what's causing it.  I can't reproduce it on my user.  I can't reproduce it on his user... unless I'm at his place, and I can reproduce it on both of his computers there.

Does anyone even know where I would start to look to diagnose this?