The conditions needed to replicate:

1) storeAsBinary enabled for keys
2) a cache store that marshals keys directly, e.g. any BucketBasedCacheStore (I'm using JdbcBinaryCacheStore)
3) a key type that is unfamiliar to MarshalledValue.isTypeExcluded(Class<?>)

If these are satisfied, then map-reduce fails with an exception like this:

org.infinispan.CacheException: java.util.concurrent.ExecutionException: java.lang.ClassCastException: org.infinispan.marshall.MarshalledValue cannot be cast to com.example.UnfamiliarKeyType
at com.example.TestMapper.map(TestMapper.java:11) ~[?:?]
at org.infinispan.distexec.mapreduce.MapReduceManagerImpl.map(MapReduceManagerImpl.java:216) ~[?:?]
at org.infinispan.distexec.mapreduce.MapReduceManagerImpl.mapAndCombineForLocalReduction(MapReduceManagerImpl.java:103) ~[?:?]
at org.infinispan.distexec.mapreduce.MapReduceTask$MapTaskPart.invokeMapCombineLocallyForLocalReduction(MapReduceTask.java:977) ~[?:?]
at org.infinispan.distexec.mapreduce.MapReduceTask$MapTaskPart.access$300(MapReduceTask.java:916) ~[?:?]
at org.infinispan.distexec.mapreduce.MapReduceTask$MapTaskPart$2.call(MapReduceTask.java:948) ~[?:?]
at org.infinispan.distexec.mapreduce.MapReduceTask$MapTaskPart$2.call(MapReduceTask.java:944) ~[?:?]
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303) ~[?:1.6.0_45]
at java.util.concurrent.FutureTask.run(FutureTask.java:138) ~[?:1.6.0_45]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:439) ~[?:1.6.0_45]
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303) ~[?:1.6.0_45]
at java.util.concurrent.FutureTask.run(FutureTask.java:138) ~[?:1.6.0_45]
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895) ~[?:1.6.0_45]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918) ~[?:1.6.0_45]