Whenever I anticipate a hotel may be booked full and I'm scheduled to arrive after 6:00 p.m., I always call the front desk (yep, I spring for the long distance call) during the day to tell them I will be arriving late and to hold my room. Some hotels ARE known to give away guaranteed rooms. It's unethical, and likely illegal, but they do it all the time. And quite often, when they're full, everyone else in the area is full as well, so they may send you across town to a hotel of far less quality than what you had reserved and would have paid for if you no-showed, even though they re-sold it to someone else.