If you are like me and will make every stupid mistake unless read about (and even then) you will appreciate this experience:
Yesterday I struggled the whole day (cold and rainy) with my focus. The lens kept buzzing and adjusting in fits and starts. I changed lenses, bought a rubber to clean electric contacts, cleared settings, took out batteries, etc. I tried everything I could think of.
Only when back home, while checking everything again, I realized that I had inadvertently set AF to AI Servo.
This ruined a lot of pix that should have been junk on their own merit.
How many more of these yet to do?