Is there a lot of free play, or 'slop' in the gear lever? This could be caused by worn gear lever bushes. There is one that sits on the bottom of the gear level and one that slides into the pivot point. If either of these are worn then it will be very easy to select the wrong gear. I don't know what the part numbers would be but I am lead to believe that they are available new through Nissan.
Failing that, I don't think you'll have many options, short of pulling the gearbox apart and trying to re-design it
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green200b - aka Alistair
The fleet:
Australia:
'80 200B GX - the
green200b - hibernating
'81 RX-7 - 12A extend port - daily
'74 RX-2 - 13B bridgeport - cruiser
Europe:
'93 200SX - CA18DET - tourer
Green200B has forgotten more about 200Bs than we will ever know.