From the game The Legend of Zelda: Link's Awakening for the Game Boy
This only works on the original classic version (not DX or the Player's Choice version, as the warp glitch was corrected in those versions).
When you see an item on-screen (Secret Seashells and Heart Pieces are the best to use this for), use the Screen warping glitch in any direction. If you perform a Perfect Warp, the item will be in the same spot on the new screen. If you then collect the item, return to the original location to find it still waiting to be gotten!
You can effectively warp in 4 directions (north, east, west, south) to make 4 duplicates of any one item. This is because the game stops rendering the visible item once it records you getting it on a single screen. However, if you want to maximize the treasures you get, warp as far as you can and collect many different copies on each screen you warp to!
How it works: The game saves the status of received treasure in its save file. Thus, each screen has a flag for 'got a Heart Piece', 'got a Secret Seashell', etc. Once those flags are up, the item won't appear in those screens the next time you visit. However, not all screens have those items - if you can trick the game into having one with the warp code, you can get the item without changing the flag on the original screen!
NOTE: If you get more than 14 Heart Containers this way, the game will think you only have one heart, making the irritating critical-life beep noise all the time.
Submitted by Artemis251