My spell is definitely bugged - I've stopped earning spell shards, and my current spell (Gem Resonance) will not activate. When I click it, the button visibly presses, but it does not activate.
I found a bug - There gets to be a point where if you buy enough of a unit that contributes towards its own discount, its price will actually start to drop. I'm going to see if I can eventually keep buying it infinitely :O
Update: I realize what's going on now - There's a rounding error with the gene upgrades. It will say you have, say, 1 gene, but you actually only have 0.9 and it won't let you spend 1.
This is now fixed, thanks for reporting :)