Virtualbox sometimes won't let go

Just minutes ago, I was installing Debian as a Virtualbox guest on a Linux Mint host when I noticed that I couldn't get my mouse cursor to move out of the box containing the Virtualbox guest.

Even having guest additions installed didn't prevent Virtualbox from not releasing my mouse, even when I pressed the right Control key, the default button used to release the mouse.

The solution

The solution that I found on the Ubuntu Forums was to simply press the Host Key, also known as the "Windows Key" as it's a key that often has a Microsoft Windows logo on it.