Hi.
If you're chaining SN (most likely applies to normal Novas as well (after the 3rd one, obviously)), and have full creature board, you get the error about full creature board. This might be a bug in itself, but I'm not sure about this.
However, if you for some reason decide that you really need that Supernova and decide to sacrifice a creature board to make room, your Supernova is converted to a Singularity in hand.
(Also, gotta love how AI on some strange occasions spam cards as crazy, like when he's going to lose - he SN-spammed (which also spawned Singularities), mindgated SoSe, played them and the card results, continuing spamming mindgate, etc. Was fun to see)