On a single system the following aggregation can't return a count > 10:

db.foo.aggregate({$limit:10}, {$group: {_id:1, count: {$sum:1}}})

On a sharded system, this can return a count of 10 * numShards.

We probably want the limit applied both on the shard (for speedup) and on the mongos (for correctness).

Luckily, this isn't likely to affect many users since $limit usually follows a $sort or a $group which both force everything after to run in mongos.