$mod treats it's divisor and remainder arguments as int values. This could lead to unexpected behaviour when these arguments are larger than int. For example:

> db.createCollection('test')
> db.test.insert([{a: 4000000000}])
> db.test.find({a: {$mod: [4000000000, 0]}})

The last query should return the single record from the test collection, but returns nothing instead.