In general, there are situations where act utilitarianism says a choice is permissible, but rule utilitarianism says the choice is not permissible.
The example I learned involved cutting across the grass as a shortcut instead of walking on a path. No one person can damage the grass, but if everyone walks across the grass, it dies, reducing everyone's utility more than gained by the shortcut.
For a real world example, I suspect that one's intuition about the acceptability of copyright piracy depends on one's intuitions about committing to pay for content and the amount of content that would exist.
In other words, it seems intuitive that the truly rational would voluntarily co-operate to avoid tragedies of the commons. But voluntary commitment to a course of action is hard to formally justify.
If everyone walks across the grass instead of on the path, that is strong evidence that the path is in the wrong place.
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.