Feeling awful

I was working yesterday, and found a nurse I was working with in a total meltdown. She had been overwhelmed all night long, and other people were trying to help her out, but her patient's required a LOT of care. She told me the breaking point was that she made a mistake and had to fill out error reports on herself. It was kinda a big mistake, but I went and looked at her patient, and I would have missed it myself. She was giving meds through an IV and they infiltrated. She didnt know that the catheter in the patient was long, so she was not assessing far enough up under the arm. I looked at the patient, and I would have missed it also, the infiltration was up under his arm. That made me very nervous that I would have missed it too.

So here is my question. How do you know things like that? How would she have known that the patient had a different catheter? How do you know "what you dont know"?

I am still pretty new and the thought overwhelms me. One of the nurse managers came in and said to her that we all make mistakes, and that she would get through it and sent her off to document. I felt like they were downplaying it a bit because she was so upset, but it made me feel like I could make a huge mistake like that and not know it.

Should I say to my boss that I would have missed it too? It is making me question how much I know. (I have been on the floor for 3 months).