We all screw up from time to time. The only bad mistake is one from which you don't learn - and end up repeating. I think anything diving or photography related is appropriate here.
I recently shot an entire dive without checking or changing the F stop on the 100 mm macro lens, which was set at 2.8. The depth of field was garbage and I ended up with a pile of bad pics that otherwise would have been ok with a greater depth of field.
The first trip using the 5D w/ 100 mm macro lens was spent with the IS turned off the entire time. Probably didn't make much of a difference, but hopefully won't happen again.
