I don't like having to save my changes every time I modify a layout. So I greatly appreciate FileMaker Pro's ability to save my layout changes automatically without constantly bugging me. You can toggle this setting in FileMaker Pro's preferences as illustrated in figure 1. But sometimes, just sometimes, I really mess things up on a layout and "Undo" isn't enough to get me out of the jam. Luckily, there's a way to revert your layout changes even if you have automatic save on.
Figure 1: Are we there yet? -- Asking if you want to save changes every time you leave Layout mode is FileMaker Pro's way of asking "Are we there yet? Are we there yet? Are we there yet?"
Tucked away at the bottom of the Layouts menu (visible only in Layout mode) is "Revert Layout." This menu item will, after confirming your wishes, revert the layout to the state it was in when you last entered Layout mode. It doesn't go any further back than that, but if you catch your mistake before switching to another mode, you can make it all go away.