When registering for a event online and choosing more than one attendee the constituents guest may be auto-matched to a constituent that you should not have access to if you are using a batch with site security and that constituent exists in another site. For example John Smith registers for a event with a Jane Smith (West site). Jane Smith does not exist in your records (West site) but does exist in another sites records (East site) in your database. When committing the batch it will register Jane Smith (East site) and John Smith (West site) for the event when that user should not have access to Jane's record.
Download and install the latest Service Pack which contains all fixes from previous patches. If you are running an older version, download and install the latest version and then the patch.