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.
Wonka's nonchalance at the bratty kids' personal safety and their parents' concern for them (along with placing priority on the sanition 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.
Someone was held hostage over a case of Wonka bars. His wife didn't seem entirely willing to pay up. It was also explicitly stated that his life was on the line.
Not only that, but since the last Golden Ticket was found shortly afterwards, the kidnappers may have assumed that their hostage's wife may have had the Golden Ticket all along and gave it to a kid she knew just to screw with them.
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.