CartridgeZelda II: The Adventure of Link was the only other Nintendo-licensed NES title to have the honor of being a gold cartridge besides the original Legend of Zelda.
ERRORInside a house within one of the towns is an NPC who simply says "I AM ERROR" when talked to. While many gamers at first found this to be a mistake in the code, there is another NPC later in the game that tells you Error knows a secret, proving that Error is his name and not an error.
It is possible that Error’s name was originally "Errol" (like the Australian-American actor Errol Flynn), but due to the letters "R" and "L" being allophones in Japanese, the programmers mistyped the name as "Error" in translating the game to English.
PlatformingAs of 2005 Zelda II is the only game in the series to use a standard jump button that doesn't require the use of a power-up, or running off the side of a ledge to clear a gap.
Town namesMany of the villages share their names with characters from Ocarina of Time (Ruto, Rauru, Mido and Saria, etc.)
Interestingly enough, Ocarina of Time is considered to be the first Zelda game continuity-wise, so it's possible the villages are named in their honour, at least in the case of the Sages (Rauru, etc.).
Information also contributed by CaptainCanuck and Mark Ennis