I am using a Nikon D5100, and for a long while (maybe a year) I have only been using M42 mount lenses, so I never used autofocus during that time.
A few days ago I've tried the only autofocus lens I own - AF-S DX Nikkor 18-55mm f/3.5-5.6 G VR, and this happened: after switching from autofocus to manual focus, when trying to take a photo, the camera sounds as if it was trying to focus (and doesn't) for as long as the button is pressed down. I am unable to take a photo then.
It will start working again after waiting for some time (maybe 15 minutes?). This happens every time I switch to manual focus, and in any mode.
A friend of mine is experiencing the same problem with her Olympus E-420 and Olympus Digital Zuiko 40-150mm f/4-5.6 ED lens.
What could be causing it?