*facepalm* I'm an idiot. I dunno how I missed both of those, my apologies. Had I seen those, especialy this gold: 'the extra note always has the same column number as the key I really pressed and a row number that differs +1 or -1' I would have responded entirely differently. I'm not sure how though :D Lemme think on it... I'm afraid that does not make it absolutely certain, at all. Sometimes when we do a mod (like adding a manual), we also make an unintentional and unknown mod (say, shorting a cable). When we see a symptom of a problem (extra keypresses), it's sensible to roll back the change (remove the new manual)... But because we didn't know we made that 'other' mod (the short), we don't roll it back. It stays, and continues to cause the symptom. sometimes the mod will cause an unintentional mod which is permanent, like shorting a chip out, or zapping it with static, and frying it. Suddenly, that $2 static strap seems like a solid investment ;) I wonder.... are you running long cables between the core and the manuals? or are the PCB traces or cables for the rows close together, or near something electronically, uhm, manipulative (a bank of relays or some mains power or transformers or something)? this sounds like crosstalk....Maybe some kinda capacitance in the switches...? Nah, then the rows would always be -1... I like crosstalk for this one. I'd say shorts between the rows, but it happens on all of them at random. It could be noise, but then neighbouring lines would be a pretty big coincidence. I'm still with crosstalk. Have you tested the voltages on the rows? Current? What kind of PCBs are you using?