This bug affects the display of textures on the stairs arranged in a certain direction (see screenshot). This bug also affects all the stairs handheld.
To reproduce this bug, you must:
¤ Launch Minecraft 1.8
¤ Use this resource pack which contains three models (in attachment)
¤ Insert stairs of all types in all possible directions.
(Sorry for the bad English, I've used Google Translate)
Your resource pack edits the model in a way that makes the texture stretch out. In other words, invalid due to a broken resource pack.