A really common concept in software development is Eat your own dogfood - aka, use the application/website/tool that you wrote as part of your day to day life. It is absolutely the number one way to find usability issues and bugs in your software. If you're writing a new calendaring tool, for instance, manage your appointments in it.
So on that topic, the number one thing that makes me wonder if web developers really "eat their own dogfood" and use every piece of websites that they've written is the presence or lack of "next page" links at the BOTTOM of webpages.
I really like 43 Places (disclaimer- I've worked with all but one of the guys who wrote it), but they're sometimes guilty of only putting next page links at the top of their webpages. If you want to, say, scroll through the 900+ people going to Sweden, you would scroll down the list, reach the bottom, and then have to scroll back up the page to click the next page link. Bad design! Next page links at the top of a page are nice, but next page links at the bottom of a list page are essential.