Many times.

One wrong keystroke and the page just.... goes away.

Or, even worse, you have a perfect page, you decide to modify it to make a different page, and instead of hitting "save as" you accidentally hit "save" and write over your perfect page with your unfinished new page.

I've learned

1. Always configure auto-save with 5 or 10 "recent versions", and a save every 5 minutes if the software supports it. I do this with all of the Adobe suite products that support it.

2. If there's no auto-save, get in the habit of manually hitting save every 5-10 minutes. You never know when the cat's going to jump up and unplug your computer, a power failure will happen, or the Blue Screen of Death will appear.

3. Backup any crucial project to another hard drive, preferably on a different computer or even better, onto a raid array such as a Terastation.

4. If you're modifying an existing document to be used for a different purpose, open it and immediately save it as the other document name to avoid the "save/save-as" problem I described above.

Even with all that in place, I still manage to screw up things and end up redoing work, but at least it's less so now.