Peter calls himself a 5th year surgical resident, but he should be a 6th year resident. He was a second year resident when the show begins in season 1. 5 seasons have passed making him a year six surgeon, ready to start his fellowship.
Benton addresses Celia Weston as "nurse" a few times and acts as if she has the medical training of a nurse. However Celia is a nurse practitioner, most nurses either have an 18-month course certificate of they are a Licensed Practical Nurse (LPN) or a two-year degree, the Associate of Nursing Science if they are a Registered Nurse (RN). Whereas a Nurse Practitioner (NP) is required to have a Masters of Nursing Science, a six-year degree. NP's have much more extensive training than LPN and RN's and they have most of the privileges doctors do including diagnosing patients, ordering lab tests and prescribing medications. Which is something Benton would know.
(as well as numerous others), reference is made to Peter Benton being a vegetarian, yet in the picnic scene following Jesse's funeral, he places a piece of fried chicken on his plate. Many people call themselves vegetarian but eat chicken and/or fish.
Peter Benton tells Maureen that he decided on a cardiothoracic surgical speciality five years prior, which would have been near the start of his residency. Yet during the course of the show he was uncertain over a specialty, and even pursued a pediatric surgery fellowship with Dr. Keaton in season 3.