You might have a product page with a few tabs like “Description”, “Specifications”, “Reviews” etc. and each of those is loaded dynamically when that tab is clicked on. Makes it possible to fit more data in one page and have use chose which one to show. I love it when websites do that.
Now if you look at the product page example I have mentioned above, if it is implemented like I have described, with all tabs being loaded once requested, search engines’ spiders cannot see their content. And this is bad. All of the sudden you have valuable, relevant and semantically related information not being in that page, which makes that page rank low for a lot of valuable keywords. I’ve seen this happen a lot.
“The right way”
<a href="big_image.jpg" class="lightbox"><img src="small_image.jpg" alt="Image" /></a>
It’s clean and clear. When like this:
You are asking for problems.
P.S. Hashbang is omitted from this post to make it less rantty. This doesn’t mean hashbangs are good. They are not.