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.
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.
Grandpa Joe is Slugworth!! It all makes perfect sense!! Think about it, what do we know about Slugworth? He's one of Wonka's longest running competitors in the candy business, and a slimy man who tries to trick children out of their prizes. Beyond that, we don't know much about him, but then consider this: what if Grandpa Joe IS Slugworth!? Here is some evidence if you're still not convinced:
Grandpa Joe is immobile until he learns about Charlie winning the Golden Ticket, an allusion to the name "Slug-worth".
The Bucket family is in poverty and Wonka is said to have beaten all of his opponents in the candy business, possibly leading Slugworth into the state he currently resides in until this opportunity pops up.
The actual Slugworth in the movie is an employee of Mr. Wonka's, and when his image flashes in the Nightmare Tunnel, Charlie is frightened while Grandpa Joe responds with "Couldn't be..." a flat out denial of the image. How would he know what Slugworth looks like? Charlie is the only one who's seen him, and Grandpa Joe is pretty quick to deflect this fear, plus, when Charlie sees the image, he cries "Grandpa!" not "Slugworth!" A stretch, I know, but then...
HE STOLE FIZZY LIFTING DRINKS!!! And convinced Charlie to help him. Why would Grandpa Joe risk his and his grandsons opportunity during this rare once in a lifetime tour to deliberately go against Wonka's rules when he's seen how it's effected the other kids? Because Joe was learning about his competitors recipes and didn't want to skip out on one.
Lastly, towards the end of the film when these actions come back on Joe and Charlie and Charlie is left without any chocolate to inherit, what does Grandpa then say as he attempts to drag Charlie out of the factory? "If Slugworth wants an Everlasting Gobstopper, he'll get one!!" The Gobstopper is Charlie's candy, so why is Grandpa Joe so sure that he'll willingly give it to Slugworth... unless he IS Slugworth!?
He also precedes the above line with an emphatic, "I'll get even with him if it's the last thing I ever do!"
"But then why doesn't Wonka recognize Slugworth?" Because Wonka has secluded himself for years, and since then, Slugworth's appearance has dramatically changed. Consider that Wonka hired Mr. Wilkinson to imitate Slugworth. Now, make Mr. Wilkinson older and remove his glasses, add a mustache and what does he begin to look like...? O_O
This is just what I was able to pick out from my own observations of the film. If you find any more clues, please feel free to share, but so far, I believe my theory is pretty sound.
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.
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.
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.