This is bugging the hell out of me. I could have sworn I had seen the challenge here: http://golf.shinh.org/all.rb a while back, but I've just gone through all 500 of those challenges and none of them are what I'm looking for.
The challenge I remember was as follows. There was a short snippet (about 10 or so lines) of code (javascript I believe) and the object was to modify the code to produce a certain output. There were a few rules that specified the types of changes that could be made. If memory serves they were to add one character, change two characters, or delete one character. If not those, then something similar. The problem had 3 or 4 solutions.
I'm aware that my description is quite vague, especially considering that I'm looking for something very specific, but I was also hoping that if not this exact challenge, someone could point me to something similar. That is, something where you have to make a couple changes to make some code work.