We've been finding that, intermittently, the Site.createPersonAccountPortalUser(userObj, newUserOwnerId, recordTypeId, password) method is retuning a null value, instead of a valid Id for a new User record. No exception is thrown; just a null value is returned.

When this happens we look at the org and find that a User record WAS inserted, as well as *multiple* Account records.

We have been unable to reproduce this using identical input data. 

Has anyone else seen this behavior?