If you stitch together photos, sometimes something that crosses the join will be in one image and not the other.

I submitted something very like this some time ago:

http://maps.google.com/maps?q=(50.333689,-4.149201)&hnea...

Make sure you have satellite imagery turned on. In this case the two overlaid images give you two copies of the breakwater.

That's the Pedro Miguel locks, second locks on the pacific side of the canal.