In the doc about mapReduce command, "mapReduce", rather than "mapreduce", is used everywhere. On mongod both names work well, since the code accepts both. But on mongos only "mapreduce" works here in the code, giving

{ "ok" : 0, "errmsg" : "no such cmd: mapReduce", "code" : 59 }

If a user alway uses mapReduce() helper, this issue will not be noticed.