Umm, we are unlikely to fix this if we can't reproduce it...
Also, and this isn't just you doing it, could people please try to make more descriptive titles for bug reports? Instead of "Snap! gets confused" a better title for this one would be "Input slot disappears in block display" or something like that.
P.S. You say you were playing with JSFunction; were you playing with the JS code for displaying scripts in the scripting area?
I just found out that putting a reporter in the ring, clicking undo, then clicking redo puts the reporter in the input list instead of back in the ring.