I had a MFU go bad...once.  MF-01 out of a Panther.  Symptom was that when powered up, the gun immediately went into a continual recoil cycle at 1/2 speed without the gun sound.  Wierd.  Tamiya replaced, no problem.  Funny thing was, my buddy Tail End Charlie had the same thing happen to one of his about 3 months later.  Oh well, at least it was easy to diagnose.