In a mongod:

> db.runCommand("ismaster")

{ "ismaster" : true, "maxBsonObjectSize" : 16777216, "ok" : 1 }

In a mongos:

> db.runCommand("ismaster")
{
"ismaster" : 1,
"msg" : "isdbgrid",
"maxBsonObjectSize" : 16777216,
"ok" : 1
}

Note the value of "ismaster": 1 vs. true.

Should be trivial to fix, and not a big deal, but it may easily lead to broken code which inadvertently does a normal "== true" or similar.