[GAMEPLAY] Game thinks Shift is being held down - Can't use any non-Shift hotkeys

In a few games since the 5.10 patch I've encountered a bug where my character cannot:

1. Level-up skills (with Ctrl-QWER hotkeys or clicking the plus sign)
2. Cast (most) skills (with QWER quickast hotkeys or clicking the ability)
3. Use the minimap to move the screen
4. Recall (with the B hotkey) or perform just about any other hotkey-based action
5. Access the menu or scoreboard with Esc or Tab respectively.
6. Also, any right-click on the ground is interpreted as an attack-move click (red animation on click similar to attack-moving as well as automatically attacking minions/champions/monsters around you). This leads to an annoying circumstance where you cannot move away from a minion wave.

It seems like the game believes that the Shift key is being held down indefinitely. It would explain just about every symptom above. Pressing Shift does not fix it, unfortunately. Same with unplugging/replugging your keyboard.

Example: I have Shift-QWER bound to self-quick-cast skills. I'm playing Leona and all of a sudden the bug occurs. Now the only skill I can cast is W as it is the only "self-quick-castable" spell she has. I cannot level-up skills, I cannot access the menu. Playing is essentially FUBAR until i reconnect.

Workarounds:

1. Reconnecting. So far the easiest workaround has been to try to reconnect. If i'm lucky, I'm out and back in within a minute.
2. Changing every keybind so that it uses Shift... Or at least setting Shift-QWER to be quickcast/normalcast spells so that your champion isn't totally gimped.

Reproducibility: I have not found a good way to reproduce this. It will happen twice in one game, then wouldn't show back up for another few games. I don't believe it is my keyboard or a stuck key as I've tried several keyboards. This has never occured to me until patch 5.10, and it's also happened to a friend on a separate computer. Both our computers are Macs, so this may be a Mac-only issue.