Decision to call you in was the right one. Big eval done, further eval of the culture wa performed and when microbiology came in at 8 AM the culture went from gram positive to staph epi, 1 tube only, and then everybody sighed a big sigh of relief and you were sent home.
If this is how it went down, as I suspect it is, it is not a medical mistake. It was a medical decision based upon imperfect information, which happens all the time in medicine (in essence, every encounter).
We don't refund money on a CT scan for abdominal pain that is later found out to be an ovarian cyst seen on ultrasound. Just another similar example. Happens all the time and is the nature of the uncertain beast that is Healthcare.