Somehow I was able to clip through blocks by walking toward them if they were at eye level and no blocks were beneath or jumping into them. 

I really don't know how this happened, but if it helps, I had Minecraft minimized for about 2 hours while my computer was still on, and then I noticed it was open and checked it out, and this happened. It was fixed after leaving and rejoining the world, but beyond that, I'm not really sure how it happened.