Comments on a previous post bring up a good point about special effects: it seems like every Hitchcock movie has a special effect or process shot that's just plain terrible. It often involves rear projection, which Hitchcock embraced enthusiastically but never used convncingly; Notorious, where rear-projection shots were used for every outdoor scene in Brazil, makes the characters look like they're standing in front of those flimsy stage backdrops from A Night At The Opera. I don't know, though, if I can think of a rear-projection shot in a Hitchcock film that's particularly bad; it's just that he did so many of them. Same with the use of studio sets in the middle of scenes otherwise shot on location: everybody did that (every John Ford movie has some studio bits picked up after the crew got back from Monument Valley), but Hitchcock did it so often and so unconvincingly that it's sort of become associated with him.
It's not like Hitchcock movies never had decent special effects. The Birds has its share of effects that don't come off, but it has plenty of effects that work (if it didn't, it wouldn't be watchable). But Hitchcock's preference for control, which meant staying in the studio as much as possible, and not turning too much of the movie over to editors and effects departments, meant that there were always going to be some bits that looked very studio-bound. Ernst Lubitsch was another control-freak director whose movies often had poor special effects. You'd think that the more controlling a director is, the more care he would take over the effects, and that's sometimes true, especially if he helps make the effects (Kubrick on 2001). Otherwise, it seems that a SFX shot almost requires the director to sit back and let someone else help direct it. Hitchcock wasn't the kind to do that.
But for me, as I said earlier, the model village at the beginning of The Lady Vanishes has always been the standout when it comes to unconvincing effects. It's so fake-looking that when the little toy car goes by -- the only sign of life in the whole village -- you wonder why they even bothered.