We are having an issue in Drupal 7.41 and civiCRM 4.6.4 (this was also happening to us with Drupal 7.34 and civiCRM 4.4) seemingly one at a time Drupal user accounts and CiviCRM accounts are becoming mis-linked to each other by one digit.
I haven't been able to determine a pattern to the users this happens to, or the timing e.g. we might get one in two weeks or two in one day. We usually get notice of this when a member cannot see their member profile field in their drupal account page.
For example, civicrm user firstname.lastname@example.org has a link to their drupal account in their contact summary, pointing to UID 8190. When I go to the drupal user account for email@example.com the UID is in fact 8189, and the link back to the civicrm contact summary simply says "profile not found".
The other symptom is that there tend to be multiple (I've seen anywhere between 1 and 15) civicrm records that are just the email address for the account that is mis-linked (for example firstname.lastname@example.org is the only information, no names or drupal uid or anything). These records seem to be created after the mis-linking. Possibly related to number of actions the person takes in drupal?
Any ideas where to start to troubleshoot/replicate this issue or has it been seen before?
I can fix it through the uf_match table or by merging the contact into a new record without transferring the drupal link and using 'sync users to contacts', but i'd like to squelch the issue if possible...