Session: Error while trying to free lock for ci_session

I have been playing around with this and have concluded the following:

1) The problem doesn't occur if I use 'files' for sessions
2) The problem does occur if I use 'database' or 'memcached', which lead me to believe it was a locking issue
3) I have many calls occurring at once (ajax) but none conflict to the data being written into session
4) If I turn off memcached file locking the number of errors drops almost completely, but not entirely
5) I cannot re-produce the problem on demand and haven't determined yet what are the circumstances in which this occurs

Thanks,

Joe