I ran into the term "exposure safety shift" in this answer. What is exposure safety shift, and what is its intended purpose?
Exposure safety shift is a feature that overrides the set aperture or shutter speed (in aperture or shutter-speed priority, respectively) in the event said aperture or shutter speed causes the other exposure determinants required to achieve a correct exposure to exceed the camera's limits.
For example, if you are attempting to shoot wide open on an f/1.4 prime lens in an outdoor environment, and the required shutter speed exceeds the camera's limits, the camera will override the f/1.4 setting you entered, changing it to a value that will permit correct exposure within the camera's limits, such as f/2.0 if the shutter speed otherwise required is one stop faster than supported by the camera.
It helps prevent images from inadvertently being exposed incorrectly when the set exposure determinant results in other exposure values that are outside of the limits of the camera, but it may also result in unexpected changes in exposure settings.
On Canon EOS cameras, it is available in the two-digit series (...40D, 50D, 60D) and higher models. It is not available in Rebels (...500D/T1i, 550D/T2i, 600D/T3i; 1000D/XS, 1100D/T3). On Pentax, it is called "Auto EV Compensation" and is available in the higher models (eg. K-7, K-5), but is not available in the entry-level cameras (eg. K-x, K-r).