GeistSR: Genesis MM2 has that bug in the hiring screen where you can browse outside of the character data and end up generating characters and items with insane stats.
At least that version is reasonably playable.
The European SNES version is so buggy that it's borderline unplayable, or at least not that fun. Instant death spells always work, damage spells do 50 damage when resisted (even if they'd do less damage if not resisted), an enemy's frenzy causes enemy attacks to fail, and the game may even get into a state where you can no longer form a party at the inn and go adventuring. (There's other things, like Max HP potions only giving 2 Max HP instead of 512.) I hear this version was planned to be released in the US, but was cancelled for unknown reasons; I highly suspect this version could not pass NoA's QA.
(Japan, incidentally, got a version of MM2 that's completely different from any other version, to the point where it may very well be an entirely different game.)
By the way, a couple other quirks of the Genesis version that I saw from watching videos:
* An enemy that saves against a a spell takes 1/4 damage instead of 1/2; this makes certain enemies, particularly the Mega Dragon, harder to kill.
* I think the Mega Dragon doesn't use its breath attack, preferring instead to eradicate one character per round.
* Enchant Item's SP cost is 50 *plus* the enchantment, rather than 50 *times* the enchantment.
(By the way, on DOS, casting Enchant Item on an item without a bonus doesn't require any SP.)
GeistSR: Genesis MM2 has that bug in the hiring screen where you can browse outside of the character data and end up generating characters and items with insane stats.
NES MM1 has a bug that allows you to change items into other items, allowing for a rather quick glitched speedrun.
Then again, NES MM1 has a pair of bugs that, when combined, will allow a single Locust Plague to wipe out your entire party; if you see a Locust Plague, kill it before it reaches the front or you might as well reset, as you're not going to get another turn once the Locust Plague attacks. (NES Locust Plagues do 255 damage (as opposed to 10 total on PC), and when an enemy kills one of your characters, it seems your party doesn't get to act for the rest of the round.)