Poor coding
Another badly coded pile of crud site to avoid is Expedia.
My credit card could not be verified for... an unspecified reason. Their form fields aren't long enough for my details. When I click on the Customer Support link, I get redirected to the home page.
I'll try and explain this in simple terms - I'm trying to pay you 352 Euros, but I can't because your site is badly coded. Reasons for failing to make a sale should not include because the customer can't actually pay using your crappy system.
Update: Apparently it's not just the Customer Service link... clicking the Hotels tab... redirects me to the home page... click on the Cars tab... redirects me to the home page. It is presumptuous of me to state, but are some of the biggest piles of coding turd written using Microsoft's technologies? Perhaps it's just a market share thing (more sites use their technologies, so there will be a higher number of bad sites, but the same proportion as other technologies) - or is it that those who can't / aren't very good, tend to choose them?