Just fixed a couple of s-t-r-a-n-g-e behaviours on my MacPro(2,1) ....

Recently, my MacPro running 10.9 began showing some alarming, but oddly cool-looking, behavior. When I opened an item from the Dock the Genie-effect animation was in slow motion -- not stuttering or jerking, but incredibly smooth, fluid slow motion. I had no idea what was causing it, but the next time I had to use the Shift key I discovered it was stuck in the depressed position. I knocked it free and the weird slo-mo went away !

So that popped into my memory a few days later when I was having a recurrence of a more annoying, and much less cool-looking, interface problem: the spinning beach ball kept popping up and taking about 15 seconds each time to go away, before popping up again just a few seconds later. Every action initiated by mouse clicks was delayed by up to two minutes, and it was really dangerous trying to do things like delete or drag files, because it was so hard to tell if the action had already registered or not, and whether I might accidentally duplicate the action, now on the wrong file, etc. One obvious thing to try was just to reboot, but when I remembered the bit with Shift key, I tried hitting that and the Caps Lock a few times, with no effect. I was having problems with mouse clicks, so I tried unplugging the mouse from the USB hub (or so I thought) and daisy-chaining it to the keyboard. After a couple of beach balls poofed out of existence, suddenly the mouse was behaving normally -- only now the keyboard was dead ! So ... it turned out I had grabbed the cord to the keyboard, not the mouse. The mouse was still plugged into the hub, but the keyboard was only connected to itself (and thus powered down). I plugged the keyboard back into the USB hub and everything was normal as could be.

So, apparently, there is something about the keyboard (and/or mouse), or about the OS's interface to the keyboard, that can be mis-set and needs to be cleared occasionally. Something to keep in the back of one's mind, and maybe save the effort of restarting the computer.