I have made this mistake before and, no doubt, I shall make this mistake again. Setting up the camera for one shot and then not changing the settings before shooting something else. I had the 200-800 lens on the camera, and I dropped the shutter speed down to 1/100^th of a second for some take off shots. This is where I left it while shooting the flying display of a couple of aircraft. A long focal length and moving target with a slow shutter speed means that there are a ton of shots that are totally unusable. That really does help in the process of deleting shots. Most of them are toast. A few did survive and that was quite a relief!