An example for an edge case is a minus sign at the beginning of a line.
- Why would anyone put a minus at the beginning of a line? - Yeah, that's just plain silly. - It must be an edge case. - Unless they really mean a list, of course. - Yeah, or signature, but it's so rare to sign your contributions on a wiki. - Yup, you use list with mutiple levels of nesting much more often.
A counterexample is the use of bold as the fist markup in a list item, just take a look at the formatting on this example page http://www.c2.com/cgi/wiki?WikiDesignPrinciples
. This is quite common and therefore not an edge case.
Programmers tend to give users all the options a user could possibly want, making it really difficult for a user to actually find what they want. Extensive menus of word processors are good examples of this.
What is the most important thing to the average computer user? They want their machine to "just work". Why does Google know how to correctly translate a United Parcel Service tracking number, while the actual UPS website requires multiple entries just to get to the point where the tracking number can be entered?
A two hour podcast that describes edge cases in detail is available at Why Software Sucks.