Some aggregation functions will turn any type into a string. This gives really strange results in an example like this:

db.foo.insert({i:1234567});   /* this is a float*/
db.foo.aggregate({$project:{_id:0, i:{$substr:["$i",0,3]}}}
{"i" : "1.2" }

Apparently it's because larger floats coerce to string of format 1.xxxe+xx

While $strcasecmp, $substr, $toLower and $toUpper all do this, only substr makes for such bizarre manifestation.