5 - You are clear to land and someone gets lost and drives onto the runway.
Had this one! A German turboprop landed at Southend on 24, when I was approaching for 06 - night, zero wind unlimited viz.
However, he turned off the runway to the right and got lost in the hangar area, instead of left onto the apron.
It was quite funny, as the controller couldn't see what had happened and I reported his position to the controller who was a bit surprised.
To cut a long story short, the pilot eventually found his way out of the hangar area and went trundling back to wards the apron, getting veeeery close to the runway.
Verbal comms were not the best by this stage and the controller (quite sensibly) sent me on my way from about 100 feet whilst he sorted out the other aircraft
So did a nice tight circuit and landed a few minutes later. Good call by the controller.