For the last few years I have been troubled with sharp and not so sharp images in a series shot with IS turned on. In one image (like the one above) feathers and features are TACK SHARP. In an adjacent image there is nothing really sharp, close, but no cigar. I ran some tests with my 300/2.8 on a tripod and found that I have about 30% unsharp images on the tripod with IS turned on and less than 10% with it turned off. That started me doing some reading again on this topic. IS and VR (Image Stabilization and Vibration Reduction) systems are designed to detect and stop small vibrations when lenses are hand held. In all cases a control system works off deviations. There must be a deviation for the system to work. Any slight movement is detected and compensated for thru the control scheme. The correction is accomplished via a moving element in the lens design. One of the elements is floating in a framework designed to counteract any induced motion in the lens. This I understand from my Mechanical Engineering days. What didn't make sense until I read in detail was this; Without a deviation or movement, the system will try to induce movement if only slightly, to make certain that it is working. Weird concept but plausible and a probable cause for the higher than expected unsharp images I have seen when shooting locked down on a tripod.
There is a great explanation on this page from Canon's Chuck Westphal. Well worth reading.
For you guys into control systems, basically the lens IS/VR is looking for an error. Any error is amplified so that a slight trend in direction can be seen and averted. Without an error the amplification keeps increasing resulting in a feedback loop going out of control and inducing movement of the floating control element in the lens. Finally, I feel better. I thought for sure I was going to have to stop drinking beer!