A query can be created to scan for a non existent key that starts with a $.

These fields are not permitted to be inserted, but can still be queried on. This leads to a collection scan, which can have many performance impacts.

If the $ character denotes an operator (like $inc, $date etc) then when some unknown operator is found, it should be rejected by the query rather than cause a collection scan. In this way queries like

db.test.find({$keyname:1})

would fail.