you know, Sushi did that a couple of times when she was a puppy. She had steps to go up and down but I think she was a baby and sometimes she felt kinda lazy to go all the way to the pad!
But she grew out of it and she NEVER makes any mistakes ever!
Just make sure you clean the sheets right away. Do you have one of those protective things on your mattress? If not, you might wanna invest in one. They are easy to wash, and prevent the pee to seep into your mattress, which would be really really hard to get rid of. If it did get to the mattress already, just soak the spor with a generous amount of nature's miracle, then let it dry.