With mongoexport 2.4.9 on Ubuntu, I get

{ "$err" : "E12001 can't sort with $snapshot", "code" : 12001 }

when running:

mongoexport -c test --query '{$query: {}, $orderby: {count: 1}}'

but not when adding a bogus query for count existing:

mongoexport -c test --query '{ $query: {count: {$exists: 1} }, $orderby: {count: 1}}'

The issue was discussed on GitHub but apparently not fully fixed, and subsequently ignored: https://github.com/mongodb/mongo/pull/207