> ensuring that knowledge isn't repeated The most fun bug I've encountered as a web developer is of this category. Two pages, both check for a logged-in user a

> ensuring that knowledge isn't repeated The most fun bug I've encountered as a web developer is of this category. Two pages, both check for a logged-in user and redirects to the other if found or not found, respectively. The bug was a subtle difference in how these were calculated, the details of which are unfortunately lost to the sands of time. The end result was that if you sat on one of the pages and waited for your user session to time out, you'd get stuck in a redirect loop between the "logged in" and "please log in" versions of the page. Anyhow, the point of this is that when you calculate the same fact two different ways, you will occasionally build something that makes an unwarranted assumption that because it's the "same fact" you wind up with the same answer. This is an entire category of easily missed and often subtle bugs.