a) Jiffy Counter dot Com: http://www.jiffycounter.com -
A simple addition of a button to copy the code presented to the clipboard enhanced conversions substantially. Apparently most users are too lazy to cut and paste.

b) Lawyer Seek: http://www.lawyerseek.com -
A simple addition of directions to "click here" let users know more clearly that they should click. Strictly speaking, this may not be a user issue, it's just a conversion principle. But I find it kind of scary that users listen to commands so well. Maybe I should think of better things to ask them to do

That's all …

Here’s more info from Wikipedia: http://en.wikipedia.org/wiki/Black_box_warning. Do I really want to take a drug that might cause heart attacks, or diabetes, or uncontrollable urges to commit suicide?
It makes me wonder if I should take drugs anymore. Does anyone feel the same way?

Here’s one example:

Breadcrumbs are a great navigational element for users. They orient the users as to where they are in the structure of a site, especially when the user comes in through a search engine to a page he or she did not navigate to. Perhaps he reached the news page associated with a topic when the main topic page was desired.

In that case, the back button just will not do. In the worst case, I’ve seen internal navigation that uses a javascript history.back() as a link that actually returns the user to the SERPs instead of to the previous page. The horrors!

Ex: http://www.yourlawyer.com/articles/read/11398/

So breadcrumbs are a good thing, right?

Well, maybe. Most breadcrumb implementations use a parameter in the URL to indicate the current category:

Ex.

http://www.lawyerseek.com/Practice/Pharmaceutical-Injury-C1/Accutane-P12/
http://www.lawyerseek.com/Practice/In-the-News-C20/Accutane-P12/
(these URLs are mod_rewritten — but that makes the argument even clearer. The parameters are after the C and after the P, respectively.
It depends on whether you have items (products, topics, etc.) that dwell in multiple categories.

If not, your life is easy, you can stop reading now.

If so, there is a major problem.

If you have a category in 2 categories, you have the same page twice, even worse would be 3-5 categories (and pages). This can lead to a penalty for duplicate content if it happens enough.

In this case you have 3 options:

a) Create the concept of a “primary” category, and exclude the other pages with a robots.txt exclusion or a meta tag exclusion.

This is the solution advocated by Dan Thies of SEO Research Labs, and the solution I use on Lawyer Seek.
b) Boost the unique content on each page (describe the category below the breadcrumb, put the category name in the title tag, different ‘also suggested’ products)

c) Use the referer header to determine the breadcrumb (very limited! but it works).

Otherwise you’ll have duplicate content. You have been warned