We're constantly getting next error (often, but NOT all the times):

OperationFailure: database error: could not initialize cursor across all shards because : too much data for sort() with no index. add an index or specify a smaller limit @ bill1shard3/shard3bill1a.ninternal.com:27018,shard3bill1b.ninternal.com:27018

BUT, during querying, we're specifying hint() with 100% existed index.
Limit, used in this particular query is 5020.

Thank you.