I ended having to learn details on how C++ work (how vtables work? why?, etc...), what some common stuff on the code was (reference-based smart pointers, locks of various kinds, thread-related stuff, alignment-based custom fast allocators, STLPort stuff, etc...), reading papers about why some things were done that way (yep, the game I choose has lots of papers and talks about its coding!), and so on.
In the end, I couldn't find any of the things I wanted to find, and couldn't convince the game IP owner to let me patch some bugs I found (that are actually killing the game review ratings on Steam), but I don't regret it one bit, I felt like I learned in the 3 months I fiddled with that, the same amount of information I needed to learn in 2 or 3 years of coding my own projects.