I have as well. I thought I may have been opening a different door through the wall or something but I don't think that is the case.

This happened to me as well. I was able to punch him and I guess I hit his face because the invisible zombie stopped hitting me for 2-3 seconds. Maybe this a bug with server loading zombie skin? I put on a pair of pants once and I looked like Ray man with no legs for like 5 minutes. Then the pants finally loaded and I was fine