Game Lag
I was about 52000 seconds into the bonus survival round when the game started lagging. I didn't want to lose my score, so eventually I sold my main tower to lose. I've got a fairly good computer, so I'm assuming lag was occurring because of the amount of things flashing on the screen including the critical strikes, adapt and amped messages (possibly also the graphics of Mr. Iron's lightning). Or the numbers were getting so high, the game couldn't handle it (my main was doing 500 trillion in damage per hit towards the end). I don't think this will be a problem for most players, but it would be depressing to lose a 50000 second run to lag.