you lose your control on the mainsheet and you get a 'zipper gybe' ... when the sail suddenly stops on 'the other side', all the sail slugs break loose, the sail grommet attachments break apart and the mainsail converts itself from being a mainsail on one side into a freaking free luff 'triangular sort of' spinnaker on the other side of the gybe. :-O

Been there done that ... and it ain't nice. Got a boom brake right after that.