Speaking of bad translations:
* In Final Fantasy 5, there is one point where you go into a tower that has two halves, one where physical attacks are the only option, and one where magic is the only option. (If you go against this, the attacks may be harshly countered, for example with multiple 9999 damage attacks or an enemy using a spell to reset the battle; during the boss fight at the top of the physical only tower, magic can't be used at all (except for the boss's final spell, but it doesn't have enough MP to cast it *anyway*). In the English PSX version, the hint is mistranslated, and gives you the same side for both instructions, resulting in you having to figure it out by trial and error (and possible game over). Also, a certain untranslatable joke was replaced with something that just doesn't work.
* The SaGa 2 remake's fan translation has its issues, including one nasty bug (game hangs when you try to soft reset it). However, the translation *also* has some grammatical mistakes, some Magic Power Cicuits (where's the 'r'?), and one English grammar mistake that was intentionally part of a bilingual joke was "fixed" in the fan translation.
* Final Fantasy Legend 2 (original SaGa 2's English version) isn't without its mistakes either, though I don't remember anything game ruining; the ones I am most aware of are spelling mistakes, particularly in the names of some high ranking monsters. However, at least it has the illegal banana trade (replaces the Japanese version's opium, I believe), and that bilingual joke was replaced with a different joke that's quite funny for English speakers. Now, excuse me while I pop an Elixier...
* In Sword of Hope, whenever an effect increases Stamina, the message says it *decreases* Stamina; in a game where certain effects actually decrease Stamina, this is a very bad translation mistake.
* In Final Fantasy Legend (English version of SaGa 1), some skills have the same name, the worst example being Gaze. There are 3 skills with the name, two being very useful (Petrify or Death), and one being essentially useless (Confuse, and before you argue that status ailments aren't useless, I should point out that confusion is bugged when it hits enemies, and will cause their melee attacks to hit your entire party). Even worse, Mutants can randomly acquire any of these three skills, and there's no way to tell which one unless you use it. It doesn't help that the game doesn't display a message when a Mutant's skill changes (so no indication that your good Gaze was replaced with the confusion one), and there's only one save slot, so you can't use a backup save to work around this. In addition, the translation messed up the durability counts of certain items (Glass Sword shattered, but it still has 49 uses left).
(Side note: I don't see any accidental typos in this post that are caught by the "post message" window.)