What do-it-yourself cautionary tales do you have?
I once had a tenant who lived next door in our duplex and thought she could hook up her own gas drier.  Smelled gas one day and couldn't find where it was coming from, so I knocked on her door.  Walked in and the whole place reeked of gas.  

Checked the drier, and sure enough, the supply line was barely even screwed on.  Asked her about it and got one of those "Yeah, it's been like that for a while now, was going to ask you about that" kind of responses.  Lucky it didn't take out the whole neighborhood.