Bubbles

There are some good arguments that bubbles don't actually exist. I find them ultimately unconvincing, but there is some merit to the idea that a bubble is just a convenient explanation rather than a useful one. At least, if you believe in bubbles, that's something to be on guard against.
I think "You can't make money doing what everyone else doing", meaning you shouldn't expect high or above market returns by doing what everyone else is doing. What everyone else is doing should pay normal, market returns. Profits, normalized to risk, should be low double digits. If you're in a situation where everyone's making tons of money, then you're probably in a bubble.