A two-fold one for the "Nightmare Tunnel" sequence:
Slugworth's appearance. The first time you see that scene, it gives the impression that the tunnel is supposed to be some kind of mind probe that shows people their greatest fears, so we assume that Charlie sees Slugworth because he was being "haunted" by the offer that he gave him. But then we learn that "Slugworth" was actually an employee of Wonka's all along...so it makes perfect sense that Wonka would be able to project his image on the tunnel.
Consider the fact that "Slugworth" works for Wonka, and that his offer of cash was really a Secret Test of Character that all four children got. Bribing kids with cash and sending a creepy dark-suited man to ambush them in dark alleys would be a good way to intimidate them into committing a crime, but so would scaring the bejeezus out of them by showing them an image of the same man in a dark tunnel filled with images of their greatest fears. Charlie proved himself worthy of inheriting the factory by proving that, not only could he not be bribed into ignoring his conscience, he also couldn't be scared into ignoring his conscience. Considering the future of Wonka's entire factory rested on that one tour, it makes sense that he would want to test the children's resolve at every opportunity.
Slugworth offers riches for a contestant to steal an Everlasting Gobstopper. However, earlier in the film, it was established that no one entered or left the factory for three years. There's no way Slugworth would have even conceived of such a prototype unless he was under the employ of Willy Wonka himself.
Related, it's implied that Slugworth approached every one of the contest winners with that offer, though the audience only saw him approaching Charlie. Given that these children came from across the United States, Germany, and the United Kingdom, it's easy to tell how he would know where they are and who they were (since they were all broadcast on television), but one other way he would have known was he works for Wonka - he would easily know which box contained the golden ticket shipment and where it would go - even down to the specific candy shop or supermarket.
We see him approaching Veruca as well. Charlie and Veruca are the only two who we actually see in the immediate aftermath of finding the ticket; the others we only see in the TV broadcasts until the day they arrive at the factory.
Wonka's nonchalance at the bratty kids' personal safety and their parents' concern for them (along with placing priority on the sanitation of his factory over their lives and the emotionlessness of his pleas for the kids to keep out of danger) makes sense if you buy into the Secret Test of Character aspect of the factory tour and the idea that Wonka knew the other kids would inevitably give in to their greed and selfishness and disobey him, and the barely-veiled contempt he has for them or their parents, Charlie and Grandpa Joe. He is gracious enough to save their lives, but has little concern for them otherwise, and is whittling them down to find his worthy heir. The only ones he genuinely cares for and disappointed over are Charlie and Grandpa Joe post-the Fizzy Lifting Drinks incident.
Each of the kids who "drop out" of the tour wear clothes that correspond to their "fates." Augustus Gloop is dressed in dark brown (falls into a chocolate river and is sent via pipe to the Fudge Room). Violet Beauregarde is wearing blue (turned into a blueberry). Veruca Salt is dressed in red (dropped into the furnace). And Mike Teavee is dressed up as a cowboy—like the ones on TV.
Wonka's entrance. At first it appears to be a mere "Oh, look at how silly Wonka is!" moment, but as Gene Wilder (who came up with the idea) stated to the director, it is a clue that you can't trust Wonka. He will lie, he will present himself in a false light... you have no idea who he truly is so you better be careful.
If you're somewhat experienced in minor SFX, you know that real glass typically isn't broken in movies, because glass is not soft. What is movie glass made of instead? Sugar. What does the Glass Elevator break through in the final scene?
Maybe this is just the result of Troping Under the Influence, but in the "Pure Imagination" song, the kids and their accompanying adults are shown to be gorging themselves on candy. Meaning that Willy Wonka was giving them no excuse whatsoever: they were given a chance to satisfy themselves entirely without a hint beyond the word of Slugworth of what might be further inside, so any further interference with Wonka operations could only be the result of a Fatal Flaw, whether an immoderate desire to consume everything, unbridled narcissism, or just the plain old greed to accept Slugworth's offer. As Mr. Wonka was looking for a successor, any child who broke the rules further in had to be more than merely hungry, or even all that curious: they had to be either willing to betray the offer of a lifetime for sheer money, meaning they were already likely greedy enough to fire or abuse the Oompah-Loompahs or otherwise cut costs in the factory in a way detrimental to Mr. Wonka's ideals, or they had to be so twisted that their desires would run Wonka's factory right into the ground. Thus making the Secret Test of Character more revealing.
The first time I saw this, I thought Wonka's instructions for the Chocolate Room made no sense. First, he lets the kids eat anything they want, then he's all, "No drinking the chocolate river!" Why let them eat everything in the room except one thing? That's like telling people they can eat the fruit from every tree in a garden except one... wait a minute...
In the movie, at least, Wonka is shown putting shoes and other clothing into his recipes. While this is played for laughs, think about it: he's planning on selling the candy this stuff is made out of to children. These are inedible objects he's putting into candy. The parents were right saying that the health department would be all over him for this kind of thing.
This presumes that he was putting them into actual cooking machines and not simply fucking with everyone.
What about the exploding candy? The recoil knocks Mike back and makes him fall over a shelf and the overall force apparently damages his teeth (we don't see it, but his mom cries out "Your teeth!" when she's examining him after the fact) and yet Wonka claims it's too weak.
On a meta level, this film's Willy Wonka has developed huge use for memes. But the brutal irony here being that a lot of what Wonka and the Oompa songs in all versions have against Mike Teevee are also very true of the meme culture. Where Mike's obsession with tv and not exploring more through reading easily translates to people being lazy on the internet instead of using it to learn and discover. A lot of these memes become a lot less funny when you realize the character in them probably wouldn't care much at all if most of the people making them suffered some Disproportionate Retribution more so than agree with you.
Veruca tends to get everything she wishes for. Most little kids tend to wish for a bunny, a puppy, a kitten, or, especially in the case of girls, a pony. Veruca obviously couldn't and wouldn't take care of them, and it's fairly doubtful her parents would know how, either. Did the pets all end up in a shelter? Even if they hired someone to feed them, would they get all they need (e.g. company for a rabbit and horse, since Veruca would most likely want it to be near her and not in a stable a while away; affection for all; training for a pony; etc.)? Most likely case is what happens way too often anyway - Kid wants a pet, loses interest after a few weeks, especially since a puppy or foal is not pre-trained and thus doesn't behave like the kid wants it to, and the poor thing ends up in a shelter.
Though the "Candyman" song seems cheerful and innocent, when learning about a certain serial kidnapper, pedophile and murderer called Dean "Candyman" Corll, it doesn't seem all that innocent anymore. Considering how flippant Wonka is about the children's safety it gets even worse.