Most browsers: You're filling out your shipping address on Amazon, and you decide you want to continue shopping. You grab a few more items and—oh no!—now you're stuck filling out that form again. Avoid this annoyance with a simple userscript.
The Simple Form Saver userscript adds a small button to the corner of your screen on form pages. After filling out the form (or during the process, just to be safe), all you need to do is click that button and it will save the entire form, as-is, for you. That way if you need to come back and you find the form empty, you can just hit the button again to have the entire page restored exactly the way you had it. It isn't the only form saver on the planet, but it is one of the best I've seen in a while.
Obviously, this isn't always a problem—things like LastPass might be able to fill in some forms for you. I've found, however, that these are never that reliable, and often only fill in a few things that I want, if that (and they aren't always accurate). And it doesn't work if you're filling out some other type of more obscure form. Simple Form Filler is remarkably reliable, since it just copies whatever you have on the page instead of trying to "guess" which boxes apply to which of its saved values. Hit the link to see what else it can do, and to try it out in your favourite Greasemonkey-compatible browser.