I've just started exploring Android Studio to write and to test my first app. I easily installed AVD emulator and tested "Hello world" on it. But then I wanted to test app also on the real hardware. I followed official instructions from here: http://developer.android.com/tools/device.html. At the end Android Studio saw my device. But I got such mistake:
Session 'app': Error Installing APK
Open terminal window and type "adb kill-server", then type "adb start-server". Usually after a few hours of inactivity, adb used to disconnect the device.