I have a person in my project team who's unreliable. He says he will do something by Friday, then when I ask him about it on Friday, I hear: "Sorry, didn't have time".
We've had many discussions about the need to signal it if something comes up, so that we can align on the priorities. I signaled that otherwise I expect him to do things we aligned on by the time we aligned on.
He's not reporting to me but he's from a vendor who we are working with and I am the project lead responsible for the project success. Although he's a contractor, he's been with the company for a long period of time and when I joined and was expected to improve the performance he wasn't happy about that, which he was very vocal about it.
Now I asked him to invite me to a meeting. The meeting was on a topic I'm accountable for. Since the topic is important I wanted to be there. He explicitly confirmed that he would invite me.
Now I ping him about the status and learn that... The meeting has just taken place.
I'm in two minds about how strong my reaction should be. On the one hand, it's a small thing. Everyone can forget. On the other hand, he didn't forget, he explained: "Oh, I thought somebody else is responsible", although I explicitly asked him to invite me. I needed to reschedule a meeting during which we were to present our results since according to what I learnt from the participants they haven't reached any conclusions.