Even with that, I've known clients come back a while later (disclaimer - I was the grunt coder, not the boss) and insist there had been verbal agreement to put in large extra features, or that the feature they'd spent some time engaging with you to specify clearly wasn't anywhere near correct adn needed completely replacing..... In one of those cases I knew exactly where the person concerned lived. Tempation.......
A lot of clients are just fine, do what they're supposed to and pay for it. A percentage will try to scam you even with an agreement if they sense an opportunity - even with what you thought was a detailed bill of work - because they realise you need them more than vice versa, they've screwed up and need the extra to cover themselves, or because they're just plain crooks. Shouldn't happen but it does; deal with it and move on.
0.a. Get a legally binding signature and include a written-only clause. Make it crystal clear that if something doesn't appear in that definition of services that everybody signed and that has a price tag on it, that it isn't covered by that price tag.
I've translated roughly 3.2 million contracts for services since switching from programming to translation, and I'm pretty sure I could survive as a programmer now, just from that alone.
0. Get an explicit agreement up front about what you're doing.
Without that, you're sunk.