Thanks very much for your replies, Clipped and and DaveReidUK. From them it sounds like there ISN'T a mechanical interlock where weight stops retraction, so it looks like the poor bugger who DIDN'T check that the pins were fitted is in the poo, regardless of whether the gear lever was accidentally activated or the electronic module controlling the retraction misbehaved
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I'm not a mechanical engineer, but such a mechanical interlock seems really easy to design. And if it malfunctioned the gear would be stuck down, which seems like the preferable failure mode. I wonder why it's not done ??