I have noticed that the rate listed at some hotels as "lowest standard rate" do not come up when trying to book. I've also noticed that (at at least one property) you can book whatever room it lets you (fully changeable and refundable, obviously) and then change the reservation online, which then lists a much larger variety of rooms, some cheaper. Obviously, YMMV.

I've also noticed a glitch where if two promo codes are selected for comparison, one of them will show no availability for the promo even if the promo is indeed available.