Heart piece duplication!! You need 5th Day to do this:
1. Save at Mountain Village with Gibdo Mask slot changed to a bottle.
2. Quit the game, get a stopwatch.
3. Start a timer when you open the game.
4. When timer reaches 1:25, catch HSW.
5. Inverted Song of Time quickly. If purple banner says 2nd/Final Day, start over.*
6. Soar to town.
7. Speak to the carpenter carrying wood.
8. Carpenter will stay in place. Put on Postman's hat and check mailbox.
9. You'll move around with the text. Hookshot the 20 Rupee chest.
10. Face the camera to the mailbox and open the chest.
11. Gold Rupee will appear on Link but you'll get a heart piece and another.
12. Do not check the mailbox. Go back in time.
13. Repeat all steps. You get two pieces each time.
Notes:
The carpenter will either have to be close enough to the mailbox, or to have the camera facing him when you check.
Tapping buttons will not work when you move around with text, so only B/X/Y can be used.
*If you take Ocarina out fast enough to not see purple banner, you'll be on 0th Day so advance to midnight before soaring out to take you to
dawn of 5th Day.
Google docs for 5th Day stuff can be viewed
here.