asterisk users - Jan 2012 - 481 Call leg/transaction does not exists Status Response

Hello All,

An Asterisk server sometimes responds to a BYE Request with a Status
481 Call leg/transaction does not exist response and is also appearing
to drop calls randomly.

What does this mean and how's one to handle it?  Does this indicate
there is some internal bug with the Server, since it appears to
completing calls without properly ending them (ie. sending out BYE
Requests to legs of call)?

Thanks,
Elliot