If you enable the user_activation operation in workflow.ini with [OperationSettings]AvailableOperationList[]=user_activation, then the registration operation will enter in an infinite loop (even if you do not connect any workflows on the pre/post trigger)

I can't figure out why when the user/register operation is re-executed in user/activate.php (called when someone click the activation link), the operation resets and start resending the activation email + disabling the account (which was just activated by the user... )