Every subsequent use of an engineering technique could be seen as a scientific experiment testing the validity of an abstract principle. It's just that by the time a principle gets to the engineering phase these experiments are no longer interesting - or they had better not be, anyway. (It would be very interesting if a bridge failed because the gravitational constant over that particular span of river were higher than in the rest of the known universe, for instance.)

Science explores the phenomenon and develops the principle. Engineering exploits the principle and provides a degree of diverse and rigorous demonstration of it. Edited to add: This process does not always occur in this order.

"I'm good at blowing bubbles with bubble gum. I have yet to charge anyone for doing it."

I think it's implied that this only applies when there is a demand for the service. Were you to find that there's a large audience for your displays, I bet you'd at least pass the hat around before doing another one.

This is a good take, but I think I like the Feynman better (which I have to assume has appeared months and months ago):

"The first principle is that you must not fool yourself, and you are the easiest person to fool."

From a different angle, there's also the Heinlein: "Your enemy is never a villain in his own eyes. Keep this in mind; it may offer a way to make him your friend. If not, you can kill him without hate — and quickly."

Or by "man."

There's the possibility that staking out a position too close to the mode (but not close enough to take those votes) will alienate a significant bloc of voters who will punish you by voting for someone else, or not at all. There's a threshold for a lot of voters where it doesn't matter that you're the "best available" candidate - for them it's like being asked to choose between a fatal dose of cyanide and one of arsenic. The fact that you're going to get one or the other is no incentive for complicity.