May be someone has a piece of code, that would work as a test for debugging abilities? We will give ability to use Visual Studio, so it should be hardly solvable by doing manual tracing and show us, that candidate is really good at debugging.
The best thing I have seen is to give the candidate a test before the interview and let them bug fix the code at the interview. Make the specs vague enough so that they will have to make judgement calls and the inputs to the problems can be vast -- say reading comma separated value data. If you want to be evil, have them modify an already existing program.
It will show you how they fix their own mistakes.
I would suggest you come up with your own code with relevance to your company's job opening. This doubles as a benefit as you have the opportunity to also evaluate a candidate's learn new concepts before attempting to solve the actual bug.
You also don't want a candidate stumbling upon this question on the internet!