Internode messages which are received by a node, but do not get not to be processed within rpc_timeout are dropped rather than processed. As the coordinator node will no longer be waiting for a response. If the Coordinator node does not receive Consistency Level responses before the rpc_timeout it will return a TimedOutException to the client. If the coordinator receives Consistency Level responses it will return success to the client.
For MUTATION messages this means that the mutation was not applied to all replicas it was sent to. The inconsistency will be repaired by Read Repair or Anti Entropy Repair.
For READ messages this means a read request may not have completed.
Load shedding is part of the Cassandra architecture, if this is a persistent issue it is generally a sign of an overloaded node or cluster.
By the way, I am on C* 1.2.4 too in dev mode, after having my node filled with 400 GB I started getting RPC timeouts on large data retrievals, so in short, you may need to revise how you query.
The queries need to be lightened