Last week I decided to give back button focusing a try. I set my 6D and 5D MkIII to focus using the back button and set the shutter to just fire away. So today I grabbed the 5D MkIII and went out for some random shots. I was going crazy trying to figure out why my blasted camera wouldn't focus. I tried switching the AF/MF switch back and forth, turned the camera on and off, and was about to remove and remount the lens. Then I remembered setting the back button focus.
D'oh!!! Luckily I had nothing critical to shoot.