Collision detection buggy?

I'm working with the Ninja Platform tutorial and have discovered a bug that I can't recreate consistently. Every once in a while, the player sprite will 'fall' through blocks beneath him.

I was trying to figure out why the sprite was still able to double-jump on a tall 'wall' after adding the additional code to prevent this, when I noticed it fell through the floor after one of my attempts. It seems to happen mostly when double jumping up a wall, but I have seen it happen after a normal jump on flat terrain. Is this a known bug, or is there a way to correct this with additional code?

(I have completed the Ninja Platformer tutorial and am using the original code with exception to the Y velocity bug being adjusted.)