The elusive 1-up loop is, while not classifiable as a cheat, one of the most effective ways to beat the system in a video game.

A 1-up loop can be found anywhere you can collect more than one 1-up in a single level. The trick is to collect the multiple 1-ups and then die.. then restart the level, collect the multiple 1-ups again, and die again.. and repeat.. and repeat. Each death gives you a net gain of lives, so if you play through the level enough times you can rack up as many lives as you want, rendering you pretty much invincible for the rest of the game.

The term "1-up loop", really, can be used to describe anywhere that a single 1-up can be collected multiple times by backtracking just after you collect it.

One example would be in the Duck Tales game, where you cannot enter the African Mines level before obtaining the skeleton key from the Transylvania level, and if you try to enter the African Mines without the key it will simply kick you over to the Transylvania level. If you enter the Transylvania level this way, however, it's possible to leave the level without beating it just by walking back out the front door.. meaning you can walk in, grab the 1-up in the mineshaft, walk back out, and repeat as many times if you want.

The 1-up loop is much less important these days, since the platformer is just about dead and infinite continue is ubiquitous (in those few games that actually make it into production that are not classifiable as sports game, fighting game, or RPG); but back in the golden days of the NES, finding a 2-up could be the greatest boon imaginable.