There’s a simple way wrong the aperture / shutter speed dials problem, and that’s to ‘lock‘ the exposure into place after you’ve set it on the camera. Most modern DSLRs will have this function (labelled as AE-L). Unfortunately, whilst a few cameras have a lock for the mode dial, it’s an unusual feature. The best way to avoid this problem is just to check before you shoot!
2. Underexposed backlit subject
If your subject is lit from behind or is darker than their surroundings, it can cause confusion for your camera. Often, you’ll find that it has exposed incorrectly and left your subject underexposed, so as to try and balance the entire exposure across the frame.