Great thread. Good to see I'm not the only DM that has failed on many levels.
The biggest failures, for me, are probably my PBEM games. My biggest success was a 3-year long Al-Qadim game that I finally decided to let go off. Was just taking too much of my time. But that was definitely the longest PBEM I ever ran, even if posts were fairly infrequent.
One PBEM I came up with was set in Planescape, and the backstory was thus:
One of the PCs was a chronomancer, and another was a priest of a Time god. The party was a group of time-traveling problem solvers, essentially. The God of time had given them a mission to stop a wizard from creating some horrific artifact that eventually would be used to alter the course of the Blood War. So, the party sets off for Mechanus to intervene with this wizard, and as luck would have it, they arrive right as the device activates. The result of this is that the PCs' bodies get dejected back to the Astral, and their souls get sucked into various magic items in the wizard's lab. This happened 50 years ago. Now the [intelligent] items are scattered throughout the planes in various owners hands. A couple of the PCs (which were all premade by me) had Githyanki swords, but were of sufficient level (15th, IIRC) to deal with any pesky Githyankis coming to claim their sword back. So, the party starts with the PCs waking up in the Astral in the holding cell of a Githyanki prison. None of them have any gear, and none of them have any idea who, or what they are.
Miserable failure.

I think it lasted all of a month. The ship got attacked by an Astral dreadnaught, and the party finds the nearest conduit out of Astral, and ends up in the pitch-black, insanity-inducing caverns of Pandemonium. I think that's as far as we got.