Is it ever acceptable to allow a PC to opt to fail? I am interested in general responses to this question; however, the question stems out of a specific example, from the game I’m currently GMing, which I could use specific help arbitrating. I have an enchanter who wants to make an enchanted sword for another character in the party, and he would like it to be cursed. Should I make it come down strictly to the role, only allow it if both players want it to happen, or perhaps not make it a failure effect at all and come up with an Ob. for the enchanter to use his skill to make cursed items?
In terms of game rules, I’d think there’d be ways to do a rush job to increase the Ob, and if the Ob > the number of dice in Enchanting, they don’t have to spend artha -> failure. Or you could Say Yes to the failure.
Enchant the item to be cursed seems to be the most straightforward answer. I wouldn’t allow a player to fail on purpose nor to dictate the results of the failure.
Yeah, “failing on purpose” is pretty inimical to BW’s philosophy, where if you succeed at a test you get what you want, and if you fail the test you don’t. If what they want is a cursed weapon, I’d say add some Ob for the curse effect (use other effects as a guideline) and test. If they fail, the weapon is in fact quite helpful to whomever they’re giving it to.
Alternately…on failure, the curse redoubles on them. 
Or the cursed blade holds the creator responsible for its loathsome existence, and will seek to harm him…perhaps an enchantment that’s only effective against the creator.
You can always be tricky with curses. Want to give a cursed gift to an enemy? The curse empowers him—it’s a curse against you! That’s a good basic mold. The curse is against the maker, not the wielder of the weapon, unless you succeed.
This seems a little bit like messing with Intents. If the player’s intent is to make a cursed weapon, they should have to actually succeed at that roll to do so, not fail at a roll to make an enchanted weapon. Perhaps failure could make you roll on the Wheel of Magic or whatever it’s called, to where instead of creating a -1D sword the player made a -1D Anvil of Brittleness or inadvertently cursed their entire house or something similar.
You know, for another perspective, you could say that the idiom of magic is that if you’re not very careful you end up with terrible cursed results. Making something cursed isn’t even a test, it’s Say Yes. You’ve just made some godawful Sword of Totally Sucking by doing a half-assed job of etching the runes and doing it without really bothering to prepare the blade. Good for you. Now what?
Presumably this is a world in which cursed weapons abound (I’m thinking D&D—Burning THAC0?), so how you get it to be useful is more interesting than its existence.
A thought off of that–
In this case, the intent isn’t to make a cursed weapon. That’s the task. I just realized that.
The intent is to sucker another character. They want to hand off the cursed weapon, so that the other PC hasn’t an inkling of what it is. “Here, have this awesome sword (heh heh heh).” You’d roll to see if you could subtly disguise the curse to be undetectable.
The intent is now make a cursed weapon that does not appear to be cursed (Does the other character have a means to detect curses anyway?).
IMHO, you don’t set up a test where the character achieves their intent by failing. We have a word for achieving your intent, success. You give them a failure condition in which interesting complications arise because they do not achieve their intent or because they get what they ask for in a way they didn’t intend.