Though it rarely happens. Their are times when you need to reset an Ubuntu system. Usually when you are setting up a new graphics card (or 2), or setting up a new xorg file. Nonetheless, if the screen freezes and CTRL+ALT+F1 or CTRL+ALT+Backspace fails to work remember these ALT+SysRq (or ALT+PrntScrn) combinations:
ALT+PrntScr+r, ALT+PrntScr+s, ALT+PrntScr+e, ALT+PrntScr+i, ALT+PrntScr+n, ALT+PrntScr+u, ALT+PrntScr+b
To remember the keys, remember this nonsensical phrase: "Raising Skinny Elephants Is Never Utterly Boring".
Alternatively, you can hold down CTRL+ALT+SysRq while very slowly typing; R E I S U B. A way to remember this is by inverting the word: "BUSIER" or remebering this nonsensical phrase: "Restart Even If System Utterly Broken".