You're absolutely correct.

Almost any reality check can fail as the subconscious mind can be very convincing in convincing one that they are not dreaming. It probably depends on the person. I've looked at clocks before in my dream, and when I looked at them again, they were exactly the same as before. I've also tried reading something and then tried again and had no problem, so for me neither of these methods work. In fact, the only method that has never failed me is to try floating. If I am dreaming I have always been able to start floating, and then know that I am dreaming, but that's just me. It is probably best to have two or three reality checks and try them all just in case one of them fails.