Problem when merging contacts which have membership records
As noted at https://civicrm.stackexchange.com/questions/27885/what-is-the-most-reliable-method-for-merging-records-with-memberships when merging two contacts when one of them has a membership, the membership does not get moved to the merged record as it should. The workaround is to ensure that the record with the membership is on the right (i.e. is the record that won't get deleted).