I do just enough development to be dangerous. Although an admin, I can not always avoid some light weight development tasks. And that means using SharePoint designer.

If you engage in inserting javascript/ jQuery inside custom pages via designer, be very careful. My hands have been burnt. I changed a default page (default.aspx or the default list page like Add/ Edit/ View). And then had to redo the entire work creating site collection from beginning again as I could not bring the page back to its original form.

So, if you are doing customization via SharePoint designer, every time you achieve a new milestone, please keep a site collection backup or a database backup. Even if you need to take multiple backups a day, it is worth it.