iPhone accidentally engages FaceTime during call This has happened to me twice recently: I have been on a call when all of a sudden I can hear the other person on loudspeaker, at which point I take the phone away from my face and observe that my face (assumably) has accidentally pressed the FaceTime button.

Is this a known issue and is there a way to prevent this from happening in the future?
This is a known issue. Apple addressed it in IOS 4.1 beta 2. Until it gets released, the simplest thing to do is talk using your headset and lock the screen (note that for some reason Apple removed the screen lock functionality without the headset).