The E3 comic with the Xbox controller from 2001-05-20http://www.penny-arcade.com/comic/2001/05/20
Instead brings up the one about waiting in line from 2001-05-18
If you move forward through the archive it takes you from the 18th to the 21st, if you move backward from the 21st, it takes you to the 20th, but with the 18th's comic.
Because 2001-05-20 (Sunday) wasn't a regular posting day, the comic instead shows up on the same page as 2001-05-18, the previous regular comic day. You can see the same thing happening with the other comics on the days prior because they posted comics almost every day that week. Also, the URLs for 5/18, 5/19, and 5/20 all go to the same page (this behavior is the same for every "off" day since PA started).