Usually happens because of mismatch between Map or Reduce class signature and configuration settings.
But also be careful about the combiner ! Check if you are using the same class as reducer and combiner. If reducer's input key-value pair is not same as its output key-value pair, then it can not be used as a combiner -because combiner's output will became input on the reducer side !
here is an example:
reducer input key val : < IntWritable, IntWritable >
reducer output key val: < Text, Text >
if this reducer is used as combiner, then the combiner will output <text, text> and reducer will receive <text, text> as input - and boom - wrong key class exception !
