Debugging parrot strings were featured on the perl6 advent calendar, day 7. Something cool about Perl 6 every day, in December.
Shows how to debug into crazy string encoding problems, when you are not sure if the core implementation, the library, the spec or the tests are wrong. It turned out, that the library and the tests were wrong.
You see how easily you could use gdb to debug into such problems, as soon as you find out a proper breakpoint.