...in several easy steps.

1: Have a too-clever-for-its-own-good CMS that is child-like in its simplicity yet Canadian-lumber-forest-like in its ability to be hacked.
2: Build several different websites using said CMS, each with their own unique hacks.
3: Have all the sites open in your FTP client and in your text editor
4: Play an episode of Firefly in the background to distrct you.

Now, the next few steps must be done in very quick succession:

5: Upload files from site A to site B
6: Realise mistake, download replacements from site C
7: Upload replacements to site D
8: Realise mistake, reset Subversion backup C
9: Upload site C to site A
10: Watch River Tam kick ass
11: Realise you're getting confused, delete everything and start again
12: Delete Subversion backup B
13: Delete entire project B

Now that you've done that, all that's left is to put everything back the way it was on A, C and D and trawl old backup disks and Google cached pages to try and get B back.

14: Watch Mal shoot someone.
15: Write it up in a post on the freshly-restored, looks-like-it-was-never-broken site B.

Fairly straightforward, really.