HTC EVO 4G camera flash will automatically disable under three conditions
Here's something I just learned about the HTC EVO camera that you might want to keep in mind if you plan on taking pictures using the dual LED flash on the back of the phone. When your battery is low, you're in cold weather, or you're talking on the phone, the EVO will automatically disable the flash.
I got this message when my battery was at 10%, but it might happen before that (you'll start getting "Please connect charger" notifications when the power level drops to 15%). I don't know what temperatures qualify as "cold weather" either and living in Hawaii, where it's 83°F right now, I don't think I'll ever find out.