Keys can be a problem because of autorepeat

There are generally several keys that are not subject to autorepeat. On A PC, both Shift keys, both Alt keys (right Alt = AltGr on international keyboards), both Ctrl keys, both Logo keys, Menu key, Shift/Caps Lock key, Scroll Lock key, Num Lock key, Pause key. Pause could be a little bit problematic because it can behave different from all other keys at the hardware level.

Polling the Shft, Ctrl, and Alt keys should be easy even through driver, operating system and toolkit layers.

Some systems may treat repeated key presses as shortcut for accessibility functions. Pressing shift five times makes Windows 7 (and others?) switch on "Sticky keys", keeping Num Lock pressed down for five seconds may enable "Toggle keys", keeping right shift pressed down for eight seconds enables "Filter keys". So you may want to use Ctrl or Alt on Windows instead, to prevent that from happening without reconfiguring Windows.

Alexander