I once worked on a game for the Wii using a 7 year old engine originally developed for the PS2. The company had a high turnover rate and the code was atrocious. The engine had been used for several games and each title introduced a new hack in the engine. You had the tipical spaghetti with meatballs mess with 2000+ lines functions and hacked code with comments like: "This code is for the E3 demo of the game xxx". Once I had to extend a 1000+ line switch statement.

But the worst I saw was something like this:

struct weapon { #ifdef xxx #include xxx #else ... #endif }