Passing a parameter by reference for an RPC does not seem to be a supported operation, and UnrealHeaderTool will already throw an error if a networked UFUNCTION includes any OutParms. However, users can still select the "Pass-By-Reference" checkbox for inputs to replicated blueprint events, creating RPCs with OutParms. There is some handling for OutParms in the NetDriver with NetDriverInternal::CopyOutParametersToLocalParameters, but if an input to a multicast RPC has "Pass-By-Reference" checked, this can lead to memory issues and crashes if the RPC is sent to more than one client.
See linked UDN with repro project
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-130854 in the post.