You can't walk onto stairs when there is a block in a certain place unless you are looking in the direction of the stairs

This behavior was only introduced in 14w30a.

Normally, you can walk up stairs as long as you are moving towards them. However, because of this bug, when a block is in a certain location, you are unable to walk up stairs unless you are looking in the direction they are facing.

I made a quick video to show the bug in action because it is difficult to explain. http://youtu.be/ywTSkQtiltk