Normally when you go to equip something in Creative, there are grey boxes indicating where you can put things (screenshot ending in 18.00.35). However, equipping a Steve head makes the other boxes, other than the shield slot, turn black (screenshot ending in 18.00.40). You can still equip things in those slots (screenshot ending in 18.00.45). This is true for only the Steve head - I was messing around with equipping heads, and this was the only one that did this.