I chased a similar bug a lon time ago, we were putting Unix on a late '80s era platform (which I wont name), some system's keyboards failed if you used the flop

I chased a similar bug a lon time ago, we were putting Unix on a late '80s era platform (which I wont name), some system's keyboards failed if you used the floppy, but not others. Turns out the floppy code sat in a tight loop polling the clock in a timer chip whenever it waited for sectors to pass by, when the hardware did this the clock to the keyboard controller changed (got faster) - turns out some keyboard chips had old firmware (they swore they didn't) that couldn't tolerate the faster clock. A couple of no-ops in the floppy loop fixed it. As someone point out above metastability issues are the real impossible bug - but the hardware guys (I wear both hats) should have got that right in the first place