I ran the following mapReduce query and the output was incorrect. I received the results shown below. If you remove the "createShards();" call in the repro, you'll see that you get the expected results. Similarly, if you change the x values in the tagged documents to all be on the same shard you also get the expected results.
Received:
"results" : [ { "_id" : "a", "value" : { "count" : 2 } }, { "_id" : "b", "value" : { "count" : 2 } }, { "_id" : "c", "value" : { "count" : 2 } } ],
Expected:
"results" : [ { "_id" : "a", "value" : { "count" : 2 } }, { "_id" : "b", "value" : { "count" : 3 } }, { "_id" : "c", "value" : { "count" : 3 } } ],