Deliberate mistake: Foils, rapiers and other fencing weapons did not appear before around the 16th/17th century, yet the main characters (in an approximate 8th to 10th century setting) use them extensively. I know that back in the days when the movie was made, the swashbuckling genre was quite popular, but it still is a reality error.
Deliberate mistake: When Hawkins mocks Ravenhurst by pouring himself some wine and then drinking it while fending off Ravenhurst's, Danny Kaye repeatedly uses the same fencing movements.
Answer: I watched the clip on YouTube, stopping and rewinding it, but I cannot see what looks like a face inside the helmet. Maybe it's just the way light and shadows are reflecting on the metal.
raywest ★
You can see it as the helmet flies to the ground. Try slowing the video down if that helps. I can't say this is the reason, as I haven't done any research, but if I had to venture a guess, the helmet needed something to sit on so that it wouldn't fall off the armour early. They probably needed a mannequin strong enough to survive the horses' jerking around, but weak enough that the mace could decapitate it.