When typing into a text box field and navigating back Safari will actually ask you if you meant to do that. It is the same behaviour when you close a tab that you've typed in, or close a window that you've typed in.
True, Safari will present a warning dialog if you have entered text and you attempt to close the current tab. It will not do this if you initiate a Back action (easily verifiable with this very site).
The latter behavior is fine in most cases, as you can initiate a Forward action and return to the page with your text. There is a specific case where this is broken, which is what I was talking about.
