This will probably happen to everyone at least once. You go to make setting changes to something (for example the IIS 6 metabase.xml file) and the changes don't work. You then have to roll back you changes. The problem, you did not make a backup first. Now you have to manually remove your changes. This process could take considerable amount of time depending on the number of changes. Especially if you were doing your editing in something that does not have undo.
Here is what happened to me. I was doing some edits to the metabase.xml file for IIS 6. For those that do not know it is the settings file for IIS. I opened the file in wordpad and made the changes necessary, saved the file then closed wordpad. I then tried starting IS and it did not start. Upon checking the event log I discovered that there was an issue with the metabase file. For some reason there were a bunch of odd characters everywhere in the file. I started to go though the file and tried to remove them. I then discovered that the odd characters, in some cases, replaced characters. I started running into occurrences where I could not figure out what the character should be. At this point I knew I was screwed.
I had no backup of the file and to make it worse I was working on a production server. The 15 seconds of downtime is now up to 10 minutes. I tried to launch the IIS administrator and it would not start. Presumably because of the now corrupt metabase.xml file. So, I could not get to the part in the GUI where I could possibly do a restore. I knew there was an auto-backup file but I did not know how old it was. I did some digging and found out that IIS creates a backup of the file when a setting via the GUI is altered. luckily I had just made a change via the GUI so there was a backup.
I did some searching on the net and figured out where I could find the auto save file. I manually restored the file and IIS started normally. Finally, after a total of 20 minutes of downtime everything was running again.
I got severely saved by an auto-backup out of pure luck. I could have literally spent hours trying to fix the corrupt file if there was no backup.
So, as a suggestion, before you make settings changes ALWAYS make a backup first (if possible).