I've been noticing what I think in an odd focus plane in the occasional photo that last 4 months. I was wondering if someone knew what was going on.

Here is an example from Sedona AZ:

Slightly cropped image:

small-9260 by kayaker72, on Flickr

Left of center (what I consider to be sharp):

small-9260-3 by kayaker72, on Flickr

Right of center (what I am concerned about):

small-9260-2 by kayaker72, on Flickr

This is happening infrequently, but I have probably seen it ~10 times in the last 4 months. All shots were taken with the EFS 15-85 on the 7D. This particular shot was @24 mm, 1/200, f/4.5, ISO 100. I know that f/4.5 could produce a limited DOF if I focused within ~30 ft, but it is more that the focal plane seems angled that concerns me. Is it normal and I just haven't noticed it before? A loose element? Any thoughts?

Thanks,
Brant