Confirmed in Trainer.
- Filled all creature slots on my side
- Played enough Supernova to get into Singularity danger zone
- Tried to play a second Supernova that turn
- Got the "there are no creature slots available" message
- Couldn't play the Supernova; no quanta gained from trying to play the second Supernova
- Ended turn
- The next turn, Supernova still in my hand turns into an upped Singularity
and the next time my deck image refreshed
Nothing happens to your deck afterwards.