Land border crossings are a little easier then the airports, but the rules are the same regardless.
They have the option to fine you should they want to (can't find the exact official web page anymore, I think it was through the state department one). They very rarely do. Most of the time it's a stern talking in secondary and of you go.
The airline could have you board if they want to go through the hassle of getting a personal approval from border patrol (I think it evolves faxing and/or mailing but I'm not sure).
When traveling internationally always keep some sort of extra proof on you that you are a US citizen. Photo of the bio page of the passport on your phone works great but also a photo of a state ID works.
That way you can get back into the US if, for instance, you loose your passport on the plane after arriving abroad. (don't ask, just trust me on this one it happens more then you want to know)