Seems to be related to this issue:" findandmodify doesn't support positional ($) operator" => http://jira.mongodb.org/browse/SERVER-1095

When I attempt to execute a positional set operation, the following error is emitted on the console: (with same example as linked).

update EXAMPLE_ARRAY query:

{ _id: ObjectId('4c6a7075c42868719b80c2e1') }

byid exception 13048 can't append to array using string field name [$] 0ms

It works fine when I use the console. My find-and-modify command is as follows:

BSONObjBuilder b;
b << "findandmodify" << collection_name.c_str() <<
"query" << Query(query) <<
"update" << update <<
"new" << see_modified;

bool result = dbcb->runCommand(string(MONGO_DBNAME),
b.obj(),
rtn);

The query phase works perfectly well.