Complex sentence: Sometimes the source code becomes available by intendation, sometimes by leaking or mistake, such as what happened with the game engine of the "Thief" series, and sometimes fans even completely reverse-engineer it from the original program binary.
Split into simpler sentences: Sometimes the source code becomes available by intendation, sometimes by leaking or mistake, such as what happened with the game engine of the "Thief" series. Sometimes fans even completely reverse-engineer source code from the original program binary.