I guess it is what is called "jaggies", is it what you're talking about ? I can see what you mean though (like the upper part of the head then below ?). I agree about that, as I was drawing free hand and didn't pay much attention, I don't have much excuse haha. I'm not too much familiar yet with what is technically wrong and learning to do things properly though ! Thanks for this comment, as it helps me to progress 

Thank you ! I really enjoy the way of thinking behind pixel art, hopefully I'll keep improving from now on