One of the most well-known codes in gaming history: press Up, Down, Left, Right on the title screen (three Downs if playing the JP version on a Japanese console), then hold A and press Start.
This is actually a crude attempt at a line break, which can be seen by setting your hex editor's bytes per row to 16 and looking at 0x00120 inside the ROM. When playing Sonic the Hedgehog on a emulator, you might have noticed the header has a very large space formatted as:
