In a weirdly coincidental series of events, I ran into problems with both of my Macs this week. The displayed effects didn't show an apparent cause. It was only through a lucky and logical conclusion that I found the problem, which was basically the same on each computer.
I'm planning a move soon and have been packing up my home-based music studio. In the middle of cleaning up, packing boxes, and relocating my workstation, I found that I had what appeared to be a stuck key on my 16" MBP. I turned off my external keyboard, restarted the computer, but kept running into problems in usability because of an intermittent and recurring stuck key. Opening Drafts, I figured out that it was the space key as I could insert other characters into the stream of blank spaces. I even went as far as a careful blast of compressed air to the laptop's space bar, to no avail. The odd thing is that I don't use this keyboard for typing - I use an external keyboard.
Some research didn't help, and I even went as far as calling Apple Support (hey, I purchased Apple Care for a reason). While on the phone, I recalled that I had packed an old keyboard in a box. This keyboard hadn't been used in at least a couple of years - I had replaced it in 2019 with the Space Grey extended keyboard, but I did reopen the packed boxes to find it. Not only did it still have battery power, which makes it the most extended battery ever to last in an Apple device, but it was also still connected to my computer. Putting in a moving box had depressed the space key. Problem solved, and I felt like a fool. And I learned a lesson about better box loading too!
Coincidentally, yesterday morning, right after a Big Sur upgrade, my M1 MBP started not responding to mouse clicks after the machine woke up from sleep. I closed down several running apps that do activate to mouse clicks, with no improvement. However, based on my previous Bluetooth keyboard experiences, I also checked my bag (which had been sitting untouched for 4 days). Sure enough, my extra work mouse was turned on and likely touching something. Turning the power backoff solved the problem.
The moral of the story - we often look for the latest software update because our toys break. Check the obvious first. Especially if it is out of sight. Two new and weird problems caused by the same error in just a couple of days are strange and preventable. Check the basics first.