It was trivial for me to reproduce Stephen Braddy’s bug video, and it’s something I’ve noticed all the time on MacOS for the past couple of major versions of the operating system.
Sierra also introduced a couple of serious bugs with the way keyboards and trackpads are interpreted. I occasionally notice keypresses getting “stuck”, and my cursor sometimes lags when it is moved. Both of these bugs have been destructive for me: I have, more than once, deleted the wrong file, and have selected the wrong action in several applications. Luckily, undo still works.
I seem to have fewer problems with my keyboard and trackpad in the MacOS 10.12.4 beta, but I shouldn’t have to be running beta software or wait for four major versions to have reliable connections for my keyboard and pointing device. That’s fundamental to using a computer; any bugs with that should be addressed before shipping.
Alas, Braddy’s bug is not fixed in the most recent beta.