Most nonsensical ban you've played under

[Vhaidara]

A DM ruled that there'd be no xp, and anything that called out xp costs would substitute gold for xp at an exchange rate of 5gp = 1xp.

Now, I have no problem with this, per se. The problem is this was combined with a highly RAW-tolerant DM that liked to stick to the printed word, even when he'd altered a pretty elemental expectation of the game (that xp is a limited resource). Throw in that this was a campaign that started at epic and where we were encouraged to optimize, and some crazy stuff became apparent in short order.

The campaign would have lasted much longer if, when we showed him some crazy abuse of the gp for xp thing, that he'd occasionally said "no, that doesn't work." Instead, we basically drowned him in a sea of crazy optimization, to which he pretty much always gave the green light. He put on a brave face, tossing Jenova, DOOM monsters, and Hellgate: London stuff our way, among others. But by the time we got to the WH40K-verse with the Doctor in tow, everything had pretty much gone to pot. I cooked up a psion cohort that had access to an infinite power points loop and ways of generating cash for free, and that was pretty much the game. Oh, and the DWK sorcerer was possessed by a greater daemon of chaos, and got his hands on the TARDIS. That pretty much was instant TKO for the entire setting.

We were supposed to keep universe jumping, with the eventual goal of stopping Cthulu-Tharizdun Gundam thing that decapitated Ao (our original employer in the campaign), before burning the D&D multiverse to the ground. Unfortunately, daemon-DWK sorcerer got ahold of the TARDIS, which was supposed to be our way out of the WH40K-verse. That and no one wanted to get close enough for the daemon to holocaust us, because the DM had rather painted himself into the corner by making the holocaust mechanics totally game-stoppingly overpowered (no resist, no save, total soul obliteration dead, at a pretty stupidly long range, with no risk to the caster, since the caster was actually a daemon). So no one even wanted to try.

That being said, we still had one caster that had Epic Spellcasting (by the book mitigation included), and my character's cohort had Epic Manifesting. I'm sure if we'd put our minds to it, we could have countered, but we'd also ostensibly taken over an entire planetary assault by some orks (by mind-switching my cohort with the ork boss), in addition to having constructed our own little extradimensional fortress with which to carry around NI people with, if we wished (and the NI fortress was pathetically easy to create...we could easily have created more). Throw in my epic monk/wizard's crew of psi-reformed simulacra sporting sniper guns and stealth armour from Hellgate: London, and we could have probably taken down that daemon. But one of the character's was likely to die, and the campaign had reached the end of it's natural lifespan anyway (DM burnout and all).

Still, good fun while it lasted. I played a very LG monk girl that was determined to do her best to always do the right thing, in a campaign where that was occasionally totally impossible (as when Tharizdun decapitated Ao and we had to turn tail and run, abandoning the rest of the D&D multiverse to Tharizdun's soul-eating cronies). Excellent moral quandries, which are always fun when teamed up with high-optimization potential.

I've never been able to put my finger on how to describe you Phelix, but I think I have an idea now.

You're Tippy's fluffy cousin...

EDIT: Quick note since I remembered that some people dislike the immortal Emperor. This is meant as a complement.