I've had this Miata for about 13 years and never had this problem. On a road trip, I stopped by the side of the road. When I jumped into the car and started it, I found I couldn't move the car forward in first gear; the car would stall. I couldn't shift into any other gear besides first and second gear. With the shifter in neutral position, the car would go in reverse.
I had it towed to a GM dealer (hey, it was a small town) and after taking it apart, they said that someone (ie. me) must have shifted the tranny into reverse and then somehow tried to shift into first. The result was that the transmission was actually in reverse while the shifter was stuck between what the mechanic called "the rails". The guy put it all back together and it works fine. There was no apparent wear, broken or loose parts that would account for this.
Has anyone had any similar problems with old Miatas?
Yes, this is common on the 91 and earlier transmissions. You do not need to remove the transmission to repair it. The cause of this is that the nylon bushing below the shifter is worn or has deteriorated. It is a $3 part. If you did not replace it, then you will need to or the problem will happen again. Somehow with a worn bushing, the two shift rods were put in to gear at the same time. Next time that it happens,remove the shifter knob and boot. You will then see three bolts holding the shifter in place. Remove them and pull the shifter out. The bushing to be replaced is located on the very bottom portion of the shifter rod. Then jack up the car and look on the left side of the transmission about half way back. The reverse switch sensor is located there. It has two wires coming out of it. Remove that and get a #2 screwdriver that is really short. If you stick your finger in, you can feel a groove in the rod just inside the hole. The groove will be just rearward of the hole. Stick the screwdriver in and pry the rod forward until it moves and makes a click sound. Go back to the top of the trans and see if the rods are centered. If they are, then the two U shaped portions of the shift rods where the shifter bushing sits will be aligned. Place the shifter in with the new bushing and install everything back including the reverse sensor.