I had a DC who started shouting at me telling me "YOU ARE NOT LUCID" and insisting I wasn't in a dream. It then started injecting me with things to try and stop me from dreaming and using lucid powers. You can read about it here.

WakingNomad says it could have been an entity, if you believe in that kind of thing. Who knows? Although it didn't stop me from being lucid thankfully.