The joke I helped omahas pull on elfs worked, and even though he grumbled at me he did so good naturedly and was laughing while he did it (at least in person).
I think it worked for a couple of reasons. One was that Omaha (and I as well) know Elf, she moreso than me, and so when she asked my help, I trusted her to know that the joke would not be harmful to Elf. Secondly, the joke was targetted specifically at him, not cast into the 'void' where anyone and everyone could be affected by it.
The jokes which fell flat today were targetting the 'world at large', not a specific person and so could not be tailored to be what they were meant to be. To many variables involved and too many unknown headspaces.
That's the essence of a good prank (or any joke for that matter).. knowing your audience and targetting the joke so that the audience involved finds it amusing.