IMPORTANT: I have a Warp9 Ethernet Smoothstepper. That's important, sorry, I should have mentioned that.
I went at it again. Actually made it all the way home once, other times it went a few inches and stopped, or only twitched.
I set Debounce higher, it didn't help. I think I put in 100 which would be 4ms.
I changed the pin from Y Home to Y-- Limit. Then, yes, it just randomly went into Reset "Limit Switch Triggered" after a moment. Sometimes it wouldn't clear when Reset was clicked, other times it cleared for a few moments and reset. So yeah it's false-triggering the input, even without moving the motors. I changed the Y-- Limit pin to a pin used for another prox sensor that hasn't been config'ed in yet. It also threw the "Limit Switch Triggered" error from there.
I checked, the shield's drain WAS connected on the board's ground. On the sensor's side, the drain wasn't connected but usually that's not important.
I put a Fluke meter on that input and saw 4.92v DC with no twitching. I switched to AC and it was 0.000v. That's not entirely definitive but it looks unlikely to be cable noise. Yeah I should have disconnected the wire totally, it's open-collector anyhow, but I didn't think of it at the time.