https://github.com/Unity-Technologies/com.unity.multiplayer.mlapi/blob/3e3aef6aa02c2a25359898319e5bd49d3518b57b/com.unity.multiplayer.mlapi/Runtime/Messaging/InternalMessageHandler.cs#L174

Basically, if the code execution gets to the linked line, you're dead. The continuationStream is pointing to inputStreamWrapper, which is modified when HandleIncomingData is called, so recursion inside HandleIncomingData is a big no-no.