Sometimes even excellent Homer nods... not many months ago I witnessed an aircraft being cleared to land on 09R at Heathrow with an aircraft on the threshold that had lined up without clearance. As I was about to report this fact, another aircraft waiting for departure beat me to it. What followed sounded like a text book example of how to handle such an incident. The inbound aircraft had it's landing clearance cancelled, the runway incursion aircraft was told to expedite vacating and, having done so, the inbound aircraft was again cleared to land as it came over the fence, all done in a calm and authoritative way. At the time there was a NOTAM in force warning of the possibility of pilots lining up on 09R when they got to the front of the queue, assuming they had been cleared to do so.
On this occasion pilots and the controller made mistakes but it was resolved in a professional manner.