Same thing you do when you forget to bring with you a spare cassete. It comes down to how you prepare. I can hardly see a recital that takes more than three hours. All you need to do is make sure you have enough space for what is foreseable duration of your event. If you're caught by surprise, at least you have the option to delete older recordings already backed-up on your PC. If you forget the spare cassete, you're in a worse position. Either way, you're better off with a HDD camera. I'm more concerned of where did I put my old cassetes. No quick keyword search for those when I have a gust I want to show a particular recording.