If you're like me, likely you've accidentally enabled FilterKeys in Windows at least once in your lifetime. To enable FilterKeys, you have to hold in the right Shift key for at least 8 seconds -- I usually do this when I'm about to start a sentence (with a capital letter, of course) and then pause to think about what I actually want to type.
After you hold the key in for about 8 seconds, a dialog pops up (and your computer speaker beeps) asking you if you'd like to keep FilterKeys enabled or not -- you're supposed to press "OK" if you want to keep it enabled, or "Cancel" if you don't want to keep it enabled.
Well, every single time this dialog pops up, I click "Cancel" to disable FilterKeys, and something funky happens. If I type without holding down the Shift key, all letters show up in uppercase; if I type with the Shift key held down, all letters are made lowercase instead of uppercase. Oddly enough, FilterKeys apparently has nothing to do with the Shift key in any way; according to the description in Control Panel -> Accessibility Options, FilterKeys is to be enabled "if you want Windows to ignore brief or repeated keystrokes, or slow the repeat rate".
Usually I need to reboot in order to fix this -- disabling it in the Control Panel does nothing for me -- but I just discovered (by reading the StickyKeys description, actually) that if you press in both Shift keys at once, things go back to normal again.
Definitely weird. I just disabled the shortcut under Control Panel -> Accessibility Options -> FilterKeys -> Settings, so hopefully it won't be happening again anytime soon.