I always do my own fluid changes nowadays, but I haven't always done so. In high school and the first part of college, I drove an 88 Ford Ranger pickup that had a 289cid V8 from a 66 Ford Fairlane swapped in, with an Edelbrock cam, intake, and carb, Hooker headers, tons of go-fast parts. A quicky oil-change place is the reason the engine swap took place, since that's how my original 2.0L 4-cylinder engine got trashed.
I was about 16 or 17 at the time...I took the truck in for an oil change, waited, paid, and was on my way. Leaving town maybe 10 minutes later, I was halfway up a very steep hill at probably 55 mph when my oil light flashed on. Given that this hill was NOT an optimal place to try and pull over and stop, I got to the top of the hill and killed the engine before rolling it to a stop in a driveway. I glance underneath and there's oil sprayed all over the entire underside of the truck. Not good.
Since this was pre-cell phone craze, I walked to my buddy's house nearby and asked his mom if I could use the phone. I called the oil-change place and they sent a manager out. He pulled the filter and confirmed his suspicions: the kid had left the OLD gasket stuck to the engine block when he screwed the new filter on. Apparently this had been an issue with this kid in the past and the manager said he was always on his ass about it to no avail. He then threw on a new filter, filled the truck up with oil and told me to call them if it had any issues.
Right away I could hear a nasty knock coming from the engine and I knew it wasn't going to make it very long. Sure enough, the next day or so I was on the way to work when it let go in a giant plume of oil smoke. The oil-change place had the truck towed to a nearby shop to have it looked at, as if there was any doubt at all as to what had gone down.
In about the most hassle-free insurance deal I've ever been a part of, their insurance company cut me a check for around $2000, the cost to buy a rebuilt engine and the labor to have it installed. I took the cash, bought the Fairlane, and started placing my first of multiple thousand-dollar orders from the Jegs and Summit catalogs.
Since then, I'm a lot more likely to avoid those places. With my current CRX, the most babied of all my cars, I've never let a shop tough the fluids at all -- it's only seen the inside of a shop maybe twice, once to have new wheels and tires mounted, balanced, & installed; and the second time to have a wheel bearing replaced while it was getting an alignment. 99% of the time, a shop will do just fine. But if you're changing dozens of cars' oil every day, eventually your mind wanders or you are tired or in a hurry or something and you make a mistake. I just don't want it to happen on MY car again.