There is a great example of this in Buffy where they obsolete themselves. In one of the earlier episodes (where a demon is scanned into a computer and activated

There is a great example of this in Buffy where they obsolete themselves. In one of the earlier episodes (where a demon is scanned into a computer and activated) they call one of the characters at home, and the phone connects, so they no she isn't online. In one of the later episodes a character has just left the room and as they are about to call him, the camera pans to his seat where he has left his flip-cell phone.