[terrajaponica]

Also, use a versioning tool such as GIT or Subversion, this way you can easily make a diff with your... Say footer.tpl or home.tpl or socialize module.
If you break something, just run through history on files that you suspect. It could also be a CSS you just randomly added.

It's free and takes 5 mins to set up (I like tortoiseSVN myself, ok it's old but it does all the job with a right click).

Don't modify and shoot in the dark like this. If you make a modification and notice 1 week later that something goes wrong you are screwed.
I'm not asking to make scheduled functional testing, which is an heavy thing to set, but at least versioning and at some point some unit tests.