Also getting "Assertion: 10338:Invalid use of reserved field name" when exporting and importing a very specific collection.
Repro

MongoDB shell version: 2.0.0
connecting to: test
> db.test_collection.insert(

{ "_id" : ObjectId("4e6f8a8ba3f2e046ce8a5300"), "text" : "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa?", "sort_order" : 1 }

)
> db.test_collection.insert(

{ "_id" : ObjectId("4e6f8a8ba3f2e046ce8a5301"), "text" : "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa?", "sort_order" : 2 }

)
> db.test_collection.insert(

{ "_id" : ObjectId("4e6f8a8ba3f2e046ce8a5302"), "text" : "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa?", "sort_order" : 3 }

)
> db.test_collection.insert(

{ "_id" : ObjectId("4e6f8a8ba3f2e046ce8a5303"), "text" : "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "sort_order" : 4 }

)

> mongoexport -d test -c test_collection -o test.json --jsonArray
connected to: 127.0.0.1
exported 4 records

> mongoimport -d test -c test_collection --file test.json --jsonArray
connected to: 127.0.0.1
Tue Sep 13 21:00:18 Assertion: 10338:Invalid use of reserved field name
exception:Invalid use of reserved field name
:

{ "$oid" : "4e6f8a8ba3f2e046ce8a5303" }

, "text" : "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "sort_order" : 4 }]

imported 2 objects
encountered 1 error