Our post on how flies manage to land on ceilings a while ago was such a hit that it inspired me to do another fly post.
In the fall Cluster Flies head for houses to spend the winter in dormancy. When you turn on the heat, they wake up and clamber around disgustingly, often in clusters around windows.
When that happens to us, we get out ye olde vacuum cleaner.
You can read about their natural history here (they leave their winter shelter and lay their eggs on earthworm holes at this time of year. Their maggots live on earthworms).