Bugs I have seen in actual code are divided into two types: those that are due to the way code inter-operates with other code (sort of integration bugs) and tri

Bugs I have seen in actual code are divided into two types: those that are due to the way code inter-operates with other code (sort of integration bugs) and tricky bugs (usually edge cases) in some highly dense code. The first type requires the candidate to learn a lot about the existing code base. The second requires more mental work but is atypical of real work. Implementing features sounds like a good idea. Perhaps create a small well defined system which is a simplification of the real world system and let the candidate implement some additional features.