I have owned this lens for many years and love it dearly. However I have not noticed that under certain circumstances the focal length will creep.
I was asked by another rescue group to get a shot of Pepsi and Cola in a Trick of Treat venue. They wanted the dogs looking directly into the camera and wanted to be looking down at the dogs, just like I would if they came to my door in a trick or treat situation.
I seldom shoot with the camera pointed down and almost never shoot one handed. However, in this case I needed to hold a treat near the lens; using my left hand for the treat and operating the camera with my right hand.
When I pointed the camera down at the dogs, without holding the lens with my left hand, the lens would creep from wide angle to a longer focal length. It is a bit disconcerting but, since I will probably never shoot this way again, I dont really mind it.