A few days ago my Magic Trackpad started misbehaving. Had some ultra cheap batteries in it, so they got the blame. New Eveready Golds in place, it seemed to be okay – for a while. Turned off Bluetooth, then turned it back on using my Wacom tablet. Completely rebooted my iMac. Trackpad was finally okay, but now my Logitech K750 solar keyboard was misbehaving. Turned it off, then back on. Checked the settings. F-keys wouldn’t work at all. Worse yet, it was like the down-arrow was stuck. Open a window in Safari and it wanted to scroll all the way to the bottom of the page. ?????
About that time, my eyes went to a pile of papers and stuff on the corner of my desk. There peeking out of the stack was my Apple Bluetooth keyboard that is only rarely used. Could it be? Yep, you guessed it. Somehow it had been turned on. It was overriding the Logitech keyboard. Once I turned it off, all was back to normal.
So, the next time something decides to not work as expected, look around to see if it has competition. Oh, and as far as Bluetooth and USB wireless go, keep a wired mouse or trackpad handy — just in case.