I had a similar problem with my mazda miata (I know--lots different) and found out it was a fairly common thing AND found a fix that didn't require a lot of work. What happens is that if you jerk it out of 5th really fast while sitting still, the rod that moves the shift fork for 5th and reverse has enough speed and inertia to keep moving it past the detent after you've moved the shifter over in neutral. The short-term fix for that tranny was to unscrew the reverse light switch which operates off the 5th/reverse shaft, and reach in with a screwdriver and pop the shaft back in to neutral. The long-term fix involved stiffer detent springs.
for a clearer explanation go to miata.net and search for reverse.
Your trans may be a totally different design, but I doubt it.