As you can see form the pictures uploaded, the 40D often focuses on the background behind the subject (actual focus point selected is circled in black). At first I thought it might be because the some of the objects are a bit small, but it happens to big subjects as well.
These subjects are not really fast moving, and this is in bright conditions.
I use back-button focusing, lens is Sigma 17-50mm f2.8 for these pictures uploaded, however I do remember this happening to my previous lens, 50mm 1.8 II which I thought was causing these problems. AF mode was single shot.
Also another thing, sometimes if I focus and lock onto a still object, take a picture, and then ask the 40D to focus again, it shifts the focus and results in either a clearer or more blur picture, depending on luck, is this normal?
Hopefully some help can be given as this issue of bad focus is really discouraging for me, seeing so many oof photos with simple subjects.