###Logitech Control Center Issues
No one has mentioned using a Logitech keyboard or mouse, but I’d like to add to the knowledge base here that the Logitech Control Center (LCC), when installed, can sometimes cause erratic key behavior.
In my case I found that it was causing a “q” to be typed after I had issued a ⌘Q to quit an app.
After I removed the LCC, I did not have any more problems with the “q”.
If you want to see my troubleshooting, with suggestions from others, see:
What is Causing the Character “q” To be Typed?