Everyone has played RPGs, or ARPGs, hack and slash or any else game, which contains a loot-table with random drops (chance, rarity, etc)... Out of pure curiosity, which combo of game/item/drop chance/rarity has gotten under your skin, lately or in the past? Which is the most nerve wracking farming session for hunting a rare item you ever had? On which game did you waste the MOST time, trying to get that sweet, secret drop of an all powerful item (or not so powerful, but still, desired)?

My own nightmare, worst experience of ALL, like EVER, is symphony of the night. Hell, damn, trying to get that Rune Sword, the most powerful ranged weapon in the game, from the filthy paws of the one and only enemy inside the game who has a chance to drop it, a Dodo Bird, has ruined my sanity and many hours of farming and still, no drop... I never found this item, like ever and it is stopping me from completing the entire collection of all the loot found inside the game... Luck mode (99 luck + lapis lazuli) and/or double ring of Arcana, don't even seem to make a difference. God i hate killing pointlessly stuff for hours just to get ONE item and it never seems to want to drop...

What is YOUR pain in gamer's behinds?

Have you played Final Fantasy 4? You know, the game that has Pink Tails?

In the version released in the US under the title "Final Fantasy 2", they are especially a pain to acquire. The enemy that drops this item appears in only one room, appears only in the rarest monster formation in the room, and has only a 1/1024 chance of dropping the item. (In other versions (except FF4 EasyType) you can at least use a certain item to force an encounter with them.)

Also, don't forget that the game has rare summons as well. (In FF2US, I did once get the Imp summon early in the game, but that was only once over many playthroughs over the years.)

The World Ends With You.
At least you could lower your own level to increase drop rates.
Some drops had special conditions though.
I mainly used a guide for some I was curious about.

I remember grinding/reloading almost endlessly to get the special Notch's Mine level in Torchlight 2 (which itself only randomly appeared), just to get either the Diamond or Iron Sword. And speaking of which, even after several hundreds of hours sunk into that game, with many, many, many runs done both in Mapworks and in NG+, I've almost never seen a Legendary item.

And while I've not played it, I'd imagine that Destiny must have one of the worst drop rates out there, if the stuff I've seen on YouTube is any indication.

World of Warcraft during The Burning Crusade. I ran Gruul's Lair for a bit over a year before I managed to see my first Dragonspine Trophy. I finally managed to get it once Sunwell had come out, so just a bit before Wrath of the Lich King came out.
And it wasn't a question of loot priority, since for various reasons I did have that, it was a case of the item not dropping. I was raiding Black Temple and Mount Hyjal while raiding with the alts of the other characters, all while hoping for a single DST to drop.
Ah, the joys of being a raiding rogue...

I experienced the exact opposite yesterday in Diablo 3. I wanted to find a pair of The Crudest Boots for my monk (doubling my mystic ally, and stacking with my Inna's Mantra set armor for a total of 10 mystic allies). So I googled the item to find out if there were any special locations or bosses that were more likely to drop it. It was suggested to simply gamble for boots with Kadala using blood shards, as the item was fairly common. I bought one pair of boots from Kadala, and they turned out to be The Crudest Boots. I think that's the most luck I've ever had drop-wise in an RPG.

Sometimes you do get strange luck. I am playing Wizardry Gaiden 4 right now, and I happened to get a Rod of Raising rather early. This weapon, which is only usable by Priests and Bishops, is actually pretty strong as it was balanced against items of similar level, not against the other items I have been finding them. Since Priests get extra attacks at a decent rate (1 every 6 levels), I think I am going to keep my Priest a Priest even after she learns 7th level spells.

Back to the main topic, in Etrian odyssey, I gave up getting one of the items needed for the most powerful sword in the game. This item is a 5% drop from a post-game boss that is not in a convenient location. To get there, it's necessary to navigate through large portions of 2 dungeon levels, and the boss fight is a typical EO dragon fight where you need to keep using a protector's skill every round just to avoid being wiped out by the boss's breath attack. (Boosted Immunize isn't good enough here because 0 Point can instantly kill, which Immunize won't protect against.) Also, the boss has defensive skills that you need to dispel, so it's not something you can just hold down A to win, even with maxed level characters (which the game pretty much expects you to have at this point).

guild wars2, specifically if you played WvWvW mode.

In the two or so years I had that game, I never had a single epic item drop in WvWvWs (loot overall was none existant) yet in "normal/PvE" mode, you could get in ten minutes, what a WvWvWer could get in a couple of years.

I thought of another one, the Tinker Bell in Final Fantasy 5. The problem with this particular rare drop is the following:
1. It is the rare drop of a boss that you only fight once and can't re-fight after words.
2. It is not next to a save point (though not as bad as that Etrian Odyssey example).
3. The boss has a state where it becomes especially vulnerable even to instant death attacks; to have a chance of getting the Tinkerbell, you have to not kill it when it's in that state.

Also, I have an example where a rare drop is not desirable:

In the original Castlevania, in stage 11 (the flat level where birds drop those hunchback enemies), each enemy killed has a chance of dropping an axe. The thing is, if you want the boss of stage 12 to be easy, you need to hold on to the holy water that you got in stage 10, and in classic Castlevania fashion, picking up a new sub-weapon destroys the one you were carrying.

Also, in Castlevania: Symphony of the Night, when doing an ACE (Arbitrary Code Execution) speedrun, there are certain random enemy drops that need to *not* happen, or the run is over. (You also need to use the Luck Code (so you can damage boost past Death), which makes the problem even worse because it increases the drop rate.)

I spent a whole lot of time in Diablo II and later in Titan Quest looking for various stuff that I can't remember. Pointless farming... Thing is that I kept a TQ save somewhere that contains rare loot from my various runs and I'm itching to start a new character and seek more pointless stuff.. I love seeking pointless shiny stuff! :)