My favorite is when project managers try to tell you it will be easier with more people.
For some things that's true.
But for some, it really just takes a while.
I tried to explain to a PM that maybe we should ask his wife next time if she can split up her pregnancy between two women. Baby could be born in half the time.
He didn't appreciate my feedback and wrote me up for it. Probably my proudest moment.
I'm not saying this for all instances. Maybe I've been fortunate. 100% of project managers I've worked alongside looked for parallel paths, and that's their job. But once the feedback from tech SME is that these are linear dependencies, they said okay. Let's move to discuss how to communicate this to leadership and customer.
My personal experience. Your mileage obviously may vary greatly. But a PM that doesn't understand the mythical man month, isn't much of a PM. Or even a competent team member.
I've had great PMs and they understood their role well. Their job was to help communicate to the stakeholders what's needed and why it's taking so long.
They are far few in between. But when you have a good one, you can't imagine your life without one.