Revealing mistake: In the opening credits, Beefy Bert has a different hair colour to what he has in the actual episodes. This was not included in Series 3.
Continuity mistake: In the episode where Henry joins the secret club the newspaper saying that Henry is horrid again has an orange image but when soggy Sid catches them the image is green.
Other mistake: Near the beginning the dividing line in the bathroom has Henry on the right and Peter on the left but as you can see Henry would be on the left due to where his room is and where he would have to walk.
Revealing mistake: Gordon is shown to have a lisp but when he opens his mouth we can see that he has front teeth.
Suggested correction: Not all lisps are caused by missing front teeth or a gap in the teeth. A dental lisp is caused when the tongue is pushed against the front teeth. Lisps can also be caused by other forms of tongue misplacement.
Continuity mistake: Just after Henry makes the bull angry, the bull headbutts the fence causing Henry to fly off. He was holding a pair of red boots which fly out of his hands when he goes through the air. The boots land on the bull's horns. He then head butts the fence again but when he does, the boots are gone and then a second later they're back on again. After that, the boots stay on the bull's horns until he shakes them off.
Horrid Henry Tricks the Tooth Fairy - S1-E7
Continuity mistake: When Peter bring his bag's into Henry's room, if you look closely at the wall behind Peter it is pink, but in the next shot if you look to the right, the wall is green - the usual colour, it then reverts back to pink again.
Horrid Henry and the Demon Dinnerlady - S1-E8
Continuity mistake: In the initial scene, Henry is offered something resembling tomato noodles. As Henry thinks he's poisoned, he is not holding anything. In the next shot, the tray and tomato noodles are on the floor despite not being dropped.
Horrid Henry and the Demon Dinnerlady - S1-E8
Continuity mistake: In the scene where the demon dinner lady is taking all the treats for the first time, the things in Henry's lunch box change in every shot.
Horrid Henry's Birthday - S1-E14
Continuity mistake: Linda wears a green shirt underneath her red skirt. However, when she asks Henry to pass the present on, her sleeves are the colour of the skirt, rather than the colour of her shirt.
Horrid Henry's Time Machine - S1-E16
Continuity mistake: Bill turns the camera around but in the long shot it is facing them until the close up when Henry turns it back round.
Horrid Henry and the Dinner Guests - S1-E17
Other mistake: When Henry's parents run into the kitchen to get Mrs. Mossy cleaned, Henry's neck is miscoloured for a split second. (00:07:19)
Horrid Henry's Parents' Evening - S5-E19
Other mistake: When they go to Henry's parent teacher talk they get lost. They had no problems at Peter's parents evening in an earlier episode.