  Bugs in learners' programs are often the result of fundamental
misconceptions. Teachers frequently face the challenge of first having to
understand such bugs, and then suggest ways to fix them. In order to enable
teachers to do so effectively and efficiently, it is desirable to support them
in recognising and fixing bugs. Misconceptions often lead to recurring patterns
of similar bugs, enabling automated tools to provide this support in terms of
hints on occurrences of common bug patterns. In this paper, we investigate to
what extent the hints improve the effectiveness and efficiency of teachers in
debugging learners' programs using a cohort of 163 primary school teachers in
training, tasked to correct buggy Scratch programs, with and without hints on
bug patterns. Our experiment suggests that automatically generated hints can
reduce the effort of finding and fixing bugs from 8.66 to 5.24 minutes, while
increasing the effectiveness by 34% more correct solutions. While this
improvement is convincing, arguably teachers in training might first need to
learn debugging "the hard way" to not miss the opportunity to learn by relying
on tools. We therefore investigate whether the use of hints during training
affects their ability to recognise and fix bugs without hints. Our experiment
provides no significant evidence that either learning to debug with hints or
learning to debug "the hard way" leads to better learning effects. Overall,
this suggests that bug patterns might be a useful concept to include in the
curriculum for teachers in training, while tool-support to recognise these
patterns is desirable for teachers in practice.
