There have been several times that I've been holding down the mouse button to shoot (duh), then when I hit space for upgrades and RELEASE the mouse button (which happens to be over the quit buttton sometimes) IT FREAKIN QUITS! This happened again just now and my score was higher than I've gotten it before -_-
I really like the fact that clicking the 10x button will buy as many as you can afford if you can't afford the whole 10. Blocking the buy buttons as you suggest would make it less convenient in my opinion. If it really needs to be addressed, maybe add a stat indicating the total amount you can afford at any given time (if you can find the space!). Or maybe just the amount out of 10 that you can afford (only when 10x buy is activated). That way it will be easier to see what is happening when you click to buy 10 and only get 4.
Why are so many people upset about the implausibility of the gravity? What about the implausibility of a penguin destroying planets with an automatic cannon? Durr, but THAT GRAVITY IS FAKE!