Failure on CMS user deletes contribution

I'm finding that valid credit card contribution records are being rolled back because for some reason the create CMS user is failing. This is the 3rd distinct bug I have hit in the createCMSUser process which has led to this problem. In this case I can't understand what the bug is as the behaviour is almost as it two processes were changing the record. Perhaps something on the drupal side.

I'm not sure if I can track down this error as it involves interaction with the drupal code & fundamentally error-prone as a result - so have decided to focus on preventing rollback. It seems to me that simply moving the createCMS user action to outside of the transaction ->commit would change problems in the createCMSUser from being catastrophic (data loss) to merely ugly (poor user experience).