Had one acft land without a clearance a few years ago at a small airport. 5min before official air traffic service hours were due to end, night-time VMC, long day flying, tuned the radio to the wrong frequency & from the silence the crew assumed I had gone home early. Didn't broadcast on the military frequency coming in (which I was monitoring in order to get an idea of where they were- the joys of non-radar environments) nor follow unmanned joining procedures and flew straight in. Got quite a fright because the working position had my back to that particular final approach, saw the landing lights, sent out the fire & rescue assuming they had a radio failure. Pilot came up to the tower rather sheepishly after shutting down to apologise. Luckily no harm done, but definitely makes you think about the what-ifs...