Hotels that claim you don't have a reservation

I'm sorry to say that I have experienced this in Rome and Athens. Both were a pain, but not world ending. We managed to go back to our old method. We first ask the people at the hotel who have no record of our reservation. The other is TI offices who may charge for finding you a place to stay, but I guess we wanted someplace to stay more than we wanted to try to stand on principle. Neither of those places wanted my credit card number when I called them from the states which was probably the problem. Both times worked out great for us. Especially in Greece we ended up at a better less know hotel for cheaper. If you give a card, you should have the guarantee I'd think.

Since I often have to travel during the busy season, I make reservations months in advance. Then two weeks before the trip, I send an email saying sweetly that I am nervous and just want to be sure my reservation is in place. They usually respond, and I have taken care of a possible problem from home.

I ran the Hotel Front Office for a Four star Worldwide Hotel Chain for several years and never heard of or experienced this. Why would any business turn away business? (Unless maybe they were in a position of being oversold but even then, it usually balances out with cancellations and no shows)
Keep your confirmation print-out handy. Confirmation numbers are best as last names can be misspelled in the computer system.
Call to confirm your reservation, even though they claim this isn't necessary. A quick call or email isn't a bad idea. Document the name of the agent/date and time of your call or printout email reply. Or speak directly to the Front Office Manager.
Secure your reservation with a credit card. Ask questions about cancellation policies.

Or if you're really concerned, why not double book at another property? Book a second reservation and cancel it once you know your room is available upon arrival. Just don't forget to cancel the second reservation!