So theres been this bug on our login / registration / logout sequence for the past few months, that seemed to have effected a really really small number of users - but effected them nevertheless. It didn't allow people to login, or register sometimes. Logging out worked but gave them errors. It was all so strange and made NO SENSE.
Well finally, after some late night coding with a good friend, I think this bug is finally finally squashed and gone. It was such an edge case scenario ... but either way - done. YEY.