We did an import of data from SalesForce and somehow the contacts did not have the right record key. So the wrong contact records got related to. A simple safeguard would be to ensure that when a Contact and Account are related to (for instance in a Task) that the Account of the Contact is the same as the Account being related. This optional safeguard will help avoiding loading bad data.

Another thing that needs to be added is to make auto-creation of related User records OPTIONAL. In many cases it results in bogus records.