When Clegg calls Howard 'Arnold', this is reference to earlier, when Howard was training under Foggy's supervision and Howard likened himself to Arnold Schwarzenegger.
Compo runs toward a stone wall and ducks down to do a headstand. The camera tracks to the other side of the wall and a stunt man is seen doing the headstand a few feet away from where Compo ducks.
When the gang are carrying Howard, Peter Sallis (Norman) at one point calls Howard, Arnold by mistake.