So, Apple likes to change things; this much is a given. The software developers behind the operating system, OS X, are no different. They’re constantly changing the way things work from iteration to iteration of Apple’s computer software.
In Snow Leopard, when you made changes to a document and tried to close that document, you’d be asked by your Mac, in essence, “are you sure you want to do that?” and you could tell it to save the changes you made, or discard them. It was a way to let us know that there had, in fact, been changes to the document, whether we meant them or not.
In Lion, that little “feature” went away. Documents in Lion were always saved, regardless. This is a neat feature, in some ways, but it keeps you from knowing if you’ve made any unintended changes.
Luckily, Mountain Lion lets you choose the way you want it to work. If you want to have that failsafe “are you sure” save changes dialog, you can enable it. If you don’t want it, you can disable it.