Last week I was shooting with the 6D and couldn't figure out why I could not change the aperture setting on my Tokina 16-28. Every time I tried to spin the wheel, the aperture setting would read L and then go back to what it was. I could change it using the Q button and setting it there on the back of the camera, but not using the control wheel. Then I switched to my 24-105 and had the same problem. It finally dawned on me that I had inadvertently set the Lock switch on the back of the camera. I'd never done that before and had no idea what effect it had when trying to change the aperture.

Now I know. Aha!