The proximity sensor? I've been face dialing and activating Facetime a bunch when I'm in a call. Not all the time, but often enough that it's quite annoying.
iOS7 on an iPhone 5. Clean install.
This same issue was in ios initial releases. It was hard to repeat so lots thought it didn't exist. With each patch Apple would fine tune the sensitivity of the seniors and eventually it became stable. I remember when I had to adjust how I held the device or or would always mute on me, so this is nothing new and will get worked out in the end. I just hope sooner, as though I don't have this issue my wife does and it is annoying when trying the talk to her on the phone to be muted or face dialed.