Design Mistakes

Here are a couple of common site design mistakes that can affect a site's usability:

Avoid Pop-Up Windows
Generally, I don't recommend opening a new page in a pop-up browser window, as it can confuse users.

Having said that, there are occasions when opening a page in a pop-up window does makes sense.

For example, you may have links to explanations of certain entry boxes on an order form page. Opening the explanatory page in a small pop-up window may be better than sending the user to a new page and then having them click back to the order page. Make sure you state clearly that the link will open up in a new browser window.
Don't Include a Web Search Engine on Your Site
Users who conduct a search on a site's search engine are interested in finding information contained within that particular site. Don't confuse users by adding a search engine that returns results from other sites on the Web