This is a bug, plain and simple. Using .show() without a speed parameter works just fine, but using .show("slow") or .show("fast") causes the errant behavior. You see? It's by adding the parameter the problem comes to light.
I appreciate the awesome library (I'll never go back to Prototype or Scriptaculous), but I have not the expertise to fix this. I rely on the ingenuity of great JS programmers to figure this out.
Try that, then try again at an explanation or solution. If you're reply contains anything other than "That's the way it is so STFU and deal.", I'll be very appreciative. After all, if the goal is to create a wonderful JS library loved by millions, shouldn't such errant behaviors be stamped out? I'm a programmer for corporations, and I'd be very lucky if "STFU and deal" was ever an acceptable response.