Losing my coder.
I was working on a fan made remix of an ancient game around 15 or so years ago. After we put two demos out, my coder disappeared. Since I knew nothing about Game Maker (And still do not; my coding base is something entirely different), the project was useless to me, so it had to be abandoned. I still have most of the project resources from back then, including all the music I sequenced for it.