Sometimes this can even happen if the screen is in the window. One night when my husband and I had only been married for a few months and Saphirah was an only furkid, we opened one of our bedroom windows late one night for some fresh air. There was a screen on the window, and we only opened it a few inches. It was winter, and it was fairly cold outside. Saphirah was sitting on the windowsill when we heard a thump. The screen was not secure and she fell out the window! Thankfully, we lived in a one-story house. So, my husband is now outside in the pitch dark, in the winter, in drizzling rain, trying to find a black cat. She did her usual escape thing of running under one of the cars, so he was able to find her fairly quickly. Needless to say, we didn't open that window much after that.