Reminded of another one! this isn't one that comes up often, but I've seen it and its really stupid no matter how you slice it:
-Rare Item Drops Required To Finish Quests-
Offenders: FF4DS
First off, lets make one thing clear:
Rare Drops (and Steals) from enemies in RPGs are always annoying. I don't think anyone argues this. At the same time, though, they're rarely relevant, more just a nice bonus you get for being lucky or persistent. So as annoying as getting, say, the Adamant Armor in FF4, GooKingSword in BoF3, etc. are, the game sort of assumes you WON'T get them, and is legitimately fair without them, so you tend to either get them as a trophy or pure dumb luck that lets you crush the game. As a result, I won't hold this as a "critical design flaw" unless the game is balanced around getting this gear (which I can't think of any games offhand that function this way, so not gonna call it out.)
ANYWAY...
So, a game has a quest or a side quest, that nets you a decent reward at the end; nothing amazing, but something you'd aim for. You're doing everything...then suddenly the game says "You need this object." Only way to get said object? Through a rare drop from a random encounter.
How stupid this is partially depends on the rate of the random drop. If this was, say, FF6 for example, it'd be 1/8 chance and thus be more acceptable (granted, the enemy in question matters too. If we were talking about something like the Brachosaur, disregarding his difficulty, I'd give the game the finger, because 1/8 drop on a 1/16 Enemy = Go die in a ditch.)
FF4DS' Rainbow jelly is the biggest offender here. Just to continue the Namingway Quest, which I might add has been very reasonable to this point (at worst, it'd require backtracking to nab an item at a town), requires you to get a 0.4% Drop from an enemy. Oh, there's an skill that doubles the drop rate to 0.8%! ...gotten in the VERY LAST ROOM OF THE FINAL DUNGEON, so you either do the entire final dungeon twice or NG+ that shit, and your rate is still sub 1%...
Oh, there's an alternative...and that's a 1% chance from Flan Princesses. I'm really loving these odds.
I know a few other games have stuff like this but not nearly as bad as FF4dS. Nier I know did this, but handled it so much better, and mostly for quests that meant a lot less. FF4DS, it stops you from getting a number of Augments just because of this bullshit.