Lesson 1: When iPhone is in headset mode, screensaver doesn't activate and/or screen doesn't go black. So unplug your headphones from the headphones socket... but remember this only effects proximity sensor.
Lesson 2: When you are debugging on hardware, the device is connected with USB cable to laptop. This cable does charging and thus screen won't go black. Unplug the USB cable, too. Now start recalling how debugging on hardware was done before cable connection was invented. Tip: log files!
Lesson 3: Something I've learned much earlier, still good to remember. If you're debugging something audio related (like I was), check ALL volume controls. There's a possibility that something has been muted!