I am having an issue with an older iMac. The app switcher will come up and then rapidly switch between either two or all icons without pressing -. It seems as if moving the mouse is causing the switching behavior. I swapped the mouse and keyboard from another mac that is not having any issues. The mouse and keyboard from this mac work on the other without issue. I've disconnected all USB devices and switch off the WiFi. None of this resolved the issue.
So, it turns out that it was some sort of hardware issue. A simple reboot or shutdown, power on did not clear it up, but powering off the Mac and unplugging it for ten minutes then powering it back up.