I was one of the early adopters of Google AdSense section targeting technology for getting more contextual ads. Almost as soon as it came out I implemented them to highlight the key content of the site (or so I thought) for AdSense. However I made a BIG mistake.

First let's look at the relevant documentation from Google:
"To implement section targeting, you'll need to add a set of special HTML comment tags to your code. These tags will mark the beginning and end of whichever section(s) you'd like to emphasize or de-emphasize for ad targeting."

The HTML tags to emphasize a page section take the following format:

"You can also designate sections you'd like to have ignored by adding a (weight=ignore) to the starting tag:"

My big mistake was to think google ad section start was for identifying ad content on my site, which I duly indicated. I didn't realize I was getting less contextual ads as a result but if Google is to be believed it must be so.
I don't know if there was any ambiguity in the original documentation or just a silly mistake on my part. However I am normally very careful with such things and if I could make such a big mistake I am sure some others can too.
Are you clear about what the tags mean for AdSense section targeting?

Now I will wait for few weeks and see if things improve (in terms of revenue).

Update: In hindsight (after 8 hours) I am not sure if Google's documentation is correct. It could even be that Google's implemented of section targeting leaves much to be desired. I was told from very very reliable source to remove section targeting as ads went haywire after I implemented following Big G's guidelines.