I've seen plenty of spurious codes come up when aircraft are changing ssr, even with "modern" aircraft. A major glitch in our EUROCATS 2000 (Australia) is feared by procedural controllers using the same system as radar ATC. If somebody squawks a code assigned to an off-radar aircraft (even monentarily) the aircraft's symbol can (and has) move to that position (ie, may no longer be visible to the ATC controlling it,) with obvious negative consequences, untill somebody notices BAW9 doing circuits at Scone....
Stand-by, change ssr, select mode C, please.
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