If you delete only one document (using justOne option) without querying by _id, like so:

db.collection.remove({a:1}, true)

you'll get this assertion:

can only delete with a non-shard key pattern if can delete as many as we find

This is a restriction for sharded collections. See source code:
https://github.com/mongodb/mongo/blob/v2.4/src/mongo/s/strategy_shard.cpp#L1153

A warning/note should be added here:
http://docs.mongodb.org/manual/reference/method/db.collection.remove/

This limitation still exists in 2.6