Been there, more than once. The first time it happened I did an OU degree.

Second time I actually contributed to the project. What I did was to document the system from a support and coder's viewpoint. I was not looking for bugs, but to document the code profile, test coverage, how and where all the functions were called, and all the little tricks and querks needed to maintain and keep the thing going. Sure, much of that was already known to many people, but it was not documented in one place, and much of the info was out of date. Along the way I learnt a lot, found a few bugs, and areas for improvement. Thereafter, when work picked-up, the document was worth its weight in gold.