Over at the dpview forums, carlsson asks:
Please take a look at andreascarlsson.com/.../Image42.html. This image is taken with my Canon S400, with the Panorama function. What have I done wrong to get different exposures on the pictures?
I notice that the sky has many small clouds. That tells me that the sun came and went behind clouds. As you were taking the pictures, the scene became darker and brighter.
When taking panorama photos, Canon cameras lock the exposure. When the sun came out from behind the clouds, the exposure did not compensate.
In addition, I would note:
Panoramic (or stitch) photos don't work out well when there is noticeable movement of objects from frame to frame: people walking, cars driving, waves washing ashore, tall grasses waving, and so on.