First of all, Samuel Jackson's name is Stacks Edwards in the movie. The reason there is blood on the wall already is because when they re-show that part, Stacks was already shot in the head, and he was on the floor. Then, Joe Pesci shot him a couple more times while he was on the floor. So I wouldn't consider that a slipup.
I'm now going to clear up and piece together the entire Stacks Edwards getting whacked scene correctly!! There's definitely a HUGE slip up in this scene but it has nothing to do with the blood splatter on the wall . OK, Tommy puts one in his head the blood splatters and after a brief pause he dumps three more bullets in Stacks head for a total of four bullets. When it switches to slow motion the scene picks back up from right after the first bullet is in Stacks head and the bloods on the wall but this time Tommy puts five more bullets in his head for a total of six bullets. So you see the real slip up here is the two extra bullets Tommy fires in Stacks head during the slow motion reenactment not the blood being splattered on the wall too early!!