Camera lies about focus

[voodooless]

I was out today and notices some behaviour with the 16-55 f2.8. At 55mm f2.8 sometimes when you focus from near to far, or the other way around, the camera will beep and show a green focus area, but the image is clearly not in focus. When it is in that mode, it keeps missing focus at subsequent tries, rarely getting focus. A power cycle fixes this problem. I haven't found out what triggers this, but I've had it happen to me several times today.. One thing that I might suspect is the AF+MF feature. it might be that after using MF, that is misbehaves, but I cannot confirm that. In any case, it really is annoying. I'm on focus priority, Single shot, AFS, single focus point (but have seen it with area mode as well).