I've had shops do all of the following (not at the same time but each has happened before I stopped letting them change my oil):

  - fill with the wrong weight oil
  - not change the filter
  - not tighten the new filter adequately, resulting in leaks
  - install a very low quality "no-name" filter
  - overfill the oil
  - overtighten/strip threads on the drain plug.
  - leave tools under the hood.

And that's just oil changes, one of the simplest things a shop can do. Maybe it's because they put their least skilled people on it, but it doesn't incline me to trust them for anything else.