I exported our membership list a few days ago and noticed that a number of them had the user's email address field changed to that of another member. Out of about 400 subscription records, about 30 had been changed to a different address (as it happens to that of our Treasurer, so it was easy to spot). This did not affect the linked joomla user's address.
These changes cannot have been made manually because the 'Re-type your email address' field still had the original email address and the record would not have saved because they do not match.
I have corrected these but now worry that this may happen again or that there may be other changes in the email field or other fields that I've not noticed. Some changes would be difficult to spot.