Program debugging is a critical and complex activity in software engineering. Accurate and fast debugging leads to high quality software and a short time-to-market. Debugging involves a very demanding cognitive process. In a case study, we found all six levels of Bloom?s taxonomy of cognitive learning, from "knowledge" through "comprehension", "application", "analysis", "synthesis", and "evaluation". The involvement of the higher levels of Bloom?s taxonomy, such as synthesis and evaluation, indicates that program debugging is a difficult cognitive task. This fact may explain the difference between novices and experts in debugging effectiveness.