Got stuck with the unenviable task of trying to solve a .dll conflict between our software and a piece of accounting software that we recommend our customers use, that both share another 3rd party software suite's print preview functionality. And of course, a very minor difference in the version causes either to not recognize the required DLLs. The Account Software people washed their hands of it, so it was solely on us. I probably put at least 12 man hours into this.
Someone had solved it before, long ago, and never documented it. I was seriously sweating it as the problem was gnarly, the customer is huge and someone I already screwed up major for last year and generally it was not going well. Like a trained monkey I was uninstalling and reinstalling software, moving files around, registering and unregistering .dlls and reading Google. The person who basically built the installer for the 3rd party software suite no longer worked at the company, and wasn't much help when I talked to him. My mood was seriously going into the dumps.
And then today, on a whim, I deleted an entire folder worth of this 3rd party company's BS and restarted my install chain, and somehow, like magic, everything started working again. I'm so relieved that it's over. I don't really know what I did to fix it, not even on a procedural level. There's a chance my documented fix isn't entirely accurate because of all the various shit I did, so I have to set up and test on another system to verify. But christ, at least I don't have that monkey on my back anymore.