When I was in college, my father decided during one of our frequent financial chats that I should change the oil in my car myself instead of paying $25. I dutifully dragged a friend who knows about such things out to AutoZone and bought supplies in bulk for the best deal. I spent about $100 and had enough stuff for 8 oil changes or so. When we got back to my fraternity house and jacked the car up, he noticed that it was slowly leaking oil from the pan, so I went back and bought a gasket. We took it apart and found that the car was built without a gasket, but had some sort of rubberized caulk instead which needed to be scraped off of the pan and off of where it connected to the underside of the car. In the process of doing all that, i managed to bend some thing that sticks down into the oil pan.
Finally after 3 days of effort (it was February in upstate NY, and I did have classes to attend so there were lots of breaks), I got Dad to agree that I was not meant to change my own oil. I threw all of the parts in the trunk, taped a garbage bag under the car, and had it towed to the dealer for repair. They had the car back together and ready to go in 90 minutes. Between supplies, towing, a new thingy, and the oil change they charged me for, it cost $400 for me to save eight bucks.
Other than adding windshield washer fluid, I have never messed around under the hood of a car since. That experience is why I pay people to do my maintenance, including rotating the tires, and I plan to have someone install my Lil Chromies for me. I figure there are lots of parts that likely cost way more than I'd save that I don't want to put in harms way. The dealer charged me $35 for the tire rotation and $35 for the oil change. Well worth it, I'd say.