I always try my hardest to make my websites simple, beautiful and most important usable. I make mistakes in my designs occasionally obvious ones. Luckily working in a team tends to stop these mistakes making it into production, and my pride makes me test to a pretty good standard to avoid any howlers.
I currently consider TAM airlines to have the worst website ever. Now it is perhaps not technically the worst website ever made, but its design led me to take 4 hours to book 2 tickets on a direct flight from Campo Grande to Sao Paulo, something that realistically should take 15 minutes max.
Sadly I had to cancel the trip, which is only possible from the website, this again took me 2 hours and as an aside is only possible to do in Portuguese.
Booking a ticket should be a simple task, but poor form validation, lack of details retention in forms and a very poor information structure lead me to take a huge time to complete the task, leaving me utterly frustrated even after I had finished my task.
Cancelling the ticket again took a huge amount of time to complete and according to the person on the phone has to be done on the website. I did not receive any confirmation email and so phoned TAM to confirm that this had been processed successfully.
I wrote a pdf on some of the TAM website problems, but after 5 pages I decided that it would take far too long to go through even 10% of the issues with the website so I cut it short. I have included the link in case you are interested in some vaguely more detailed issues of their site (perhaps some of them are petty but seriously 6 hours spent on a site unnecessary has made me petty :) ).