An MD80 altitude select knob was unknowingly changed to a lower altitude which then armed automatically without any pilot input. The altitude set knob turned with minimum input force leading to an altitude deviation.

Narrative

During descent leaving 6000 ft we questioned the altitude pre-select value of 3000 ft. The controller responded '6000 ft.' At that time we were at approximately 5500 ft and begin a climb back to the assigned altitude. No conflict was involved. We attempted to determine how the altitude got reset to the lower value and had no positive answer. I believe that the altitude select knob somehow got bumped. This is a real problem with the MD80 aircraft. The altitude select knob is VERY easy to move; unlike any other aircraft I have ever flown whose select knobs offer some resistance. The MD80 has none. What's more; a software change was made many years ago which makes it unnecessary to pull out the knob to set the altitude. Now; when the knob is bumped; the altitude is automatically armed; rather than displaying a blank FMA. This altitude alert system has been the reason for many altitude busts in the industry; and is a very poorly thought out system. I suspect that fatigue may be a casual cause as well.Callback conversation with reporter revealed the following information: The reporter stated clearly that the current altitude capture software does not require the pilot to pull the altitude set knob in order to arm the altitude in the altitude set window. Reporter also stated that since no pilot input is required after a new altitude selection; the new altitude can be accidentally changed and armed for capture with no crew input; as happened in this case.