Further, perhaps ambiguous evidence that Harry's machinitions won't be as successful or simple as he imagines. From Chapter 20, in reference to Quirrell's insistence that Dumbledore pay for Harry's Occlumency lessons with a neutral party:

Dumbledore was frowning. "Such services are extremely expensive, as you well know, and I cannot help but wonder why you deem them necessary."

"If it's money that's the problem," Harry spoke up, "I have some ideas for making large amounts of money quickly -"

"Thank you Quirinus, your wisdom is now quite evident and I am sorry for disputing it. Your concern for Harry Potter does you credit, as well."

Dumbledore immediately identifies Harry's money-making scheme as a terrible idea (even without knowing what exactly it is) and is actually willing to compromise his prior stance merely by being reminded how ignorant and childish Harry can be.

Dumbledore is probably the number one character, except for perhaps Snape, who has demonstrated the most knowledge of Muggle technology, culture, and institutions. I think it's a good bet that Dumbledore, hearing Harry's statement, immediately realized that Harry had hatched some hare-brained scheme with all kinds of horrible consequences that were obvious to Dumbledore, with his knowledge of both worlds, but opaque to Harry.