That is called the loop conditional terminal.
If the loop is running, you won't see any errors pop up because the error wire is connected to the tunnel of the loop. Try putting a probe or indicator on that wire inside the loop and you will see whether you do have errors.
Once the loop stops, the error is passed outside the loop through the tunnel. Eventually when there is no where else for it to go, the automatic errror handler will pop up a dialog telling you the error exists. It probably existed all along. You just didn't see it because the error wire was being handled by the tunnel.
