CiviEvent backing out of contact change from dedupe rule
If you have logging on then via the ChangeLog tab on the Contact you should be able to garner some of the required info

If that doesn't help enough then go to the log_civicrm_contact/event/contribution etc tables and fire in the relevant ID and see what it has to offer

Key thing with dedupe rules is to make sure your Weights and Thresholds make mathematical sense - esp when fields like Phone and Email will be 'scored' for every match it finds - so if a contact has the same email three times, then dedupe will score 3 x email weight.

Hence it is important to have a rule like First 10 Last 10 Email 2 = Threshold 22 (ie it would have to the same email on the contact 11 times for it to count as a dedupe while ignoring First and Last) - whereas eg First 5 Last 5 Email 5 = Threshold 15 means it could match on a record that has the email 3 times (which is quite possible) even though the First and Last do NOT match.