Rookie teachers often undermine the rest of the staff. They don't mean to. Example:
There's a snack for teachers who are doing training. Kids are not allowed to have the snack, for obvious reasons: one kid gets it, why not all?
But a rookie decides to share with a kid. Now every other staff member who plays by the rules - no snack for kids - has a tougher job in saying "No."
A junior is not trying hard. A rookie tells the junior "Hey, colleges care about junior year GPA more than anything. When you're a senior you can slack off, but right now you've got to try hard."
The message is partially true. But obviously this undermines anyone teaching the kid as a senior.
It's hard for rookie teachers (and some veteran teachers, actually) to avoid the temptation to
a. Make exceptions to rules/norms
b. Attempting to curry favor by validating a student complaint about another subject ("Oooh, I hate math too"; "Yeah, I never liked Shakespeare when I was growing up.")
Antidote to temptation? An overall sense of strength/competence. That is, these tend little hiccups happen most among rookies who feel insecure about how kids perceive them.