One of the most amusing uses of source commentary is a game 'we' (co-workers and I) made up, by the name of 'The Lunch Game'.
The Premise of the game is rather simple. Our code is all modular, so a project is comprised of lots of little objects that in turn make a whole. Whenever one of us creates a new module to add on the the structure, we add a random day of the work week (Monday through Friday) to the header comments.
What happens then, after that revision is deployed and put into production, is the next time a bug is discovered in that module, the person debugging it will look at the day of the week in the comments; if that code was a) written by a programmer other than himself, and b) the day in the comments is the current day, then the person who wrote that module has to buy lunch for the one debugging it.
This has two upshots: One, in encourages extensive testing for bugs, and it encourages your attack bugs found later as fast as you can -- if it's in a module you wrote, you get out of buying lunch for anyone else, and if it's someone elses module, you get lunch bought for you.
I've 'won' twice. I've 'lost' three times.