Follow on issue from
MRUNIT-127, also relating to grouping comparators.
In the new MapReduce API Reducer's reduce method, you are passed a reference to a key and an Iterable for the values. What's not particularly clear is that as you iterate through the values, the value of the key is also changing silently. This is not usually noticeable, since the keys that were grouped often have the same value anyway, but with a custom grouping comparator this isn't necessarily the case.
This is different to the old API, where you would get the first key in the group and this wouldn't change as you iterate over the values.
MRUnit uses the "old API" style behaviour for both old and new APIs, so some unit tests may not give a consistent result to an actual mapreduce job.