Here's a funny one.....
You know it's been a long time since you last used your DSLR camera when you get it out, 2 days before leaving for France, take a few test shots, and find that you don't have any autofocus in aperture or shutter priority modes..... Works fine in automatic (which I rarely use). So I spent about half an hour going through all of the menus, checking out any custom functions that I'd set, but no, the darned thing still won't autofocus in anything other than auto mode.
Then, after some frantic Googling I just happened upon a reference to 'back button focus'...... It rang a vague bell.... And yes, many months ago when I last used the camera, I'd set back button focus on the AF-ON button..... Now I've remembered, it works fine.
Relieved.....