This is an interesting test to make.
Open a text writing application like Notepad, keep both Shift keys pressed (the left & the right one) and write the text ""THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG".
What do you get? What keyboard do you use?

I get "HE QUIC BROWN OX UMP OVER HE Y OG" and I use the Logitech Media 600 USB keyboard with Croatian layout.

As you can guess, this is a test to demonstrate keyboard jamming (a problem most keyboards have when multiple key rollover is used).
Some PS/2 keyboards, most mechanical keyboards and keyboards with antighosting on all keys do not experience this problem.