According to the documentation, only field names starting with `$` are reserved. But it seems that because of how the wire protocol is implemented (looking at what the drivers do, the documentation is a bit sparse here), certain field names take on a special meaning, such as "orderby" or "query":

> db.test.insert({_id:123, orderby: 1, query: 2});
> db.test.insert({_id:124, orderby: 2, query: 1});
> db.test.find({orderby: 1})
  { "_id" : 123, "orderby" : 1, "query" : 2 }
> db.test.find({query: {}, orderby: 1})
  error: { "$err" : "sort must be an object or array", "code" : 13513 }

The wire protocol should be updated to use some kind of different scheme to avoid ambiguity, such as prefixing the special parts of the query with $ as happens in other places.