It depends on the mistake of course (he forgets some stuff about me sometimes, usually I let that slide, he's shown enough times that he also remembers stuff).
Forgetting an appointment or not paying attention, this has never happened, but both I'd mention when we see next. If it kept on happening, I'd bring it up two or three times before saying that it keeps happening, I'll see somebody else.

My T wouldn't just dismiss it like your T seems to do. He might explain why it happened, but then he'd explore more with me what we both can do to make it better. For example one time I mentioned he talks a bit much sometimes. He explained that this is one of his issues, he feels uncomfortable with silence. Then asked me why I'd like to talk him less, in which situations it's okay for him to just talk and when he should maybe be a bit more quiet. And since then he's never talked enough to bother me again.

If you'd like to not switch Ts and give her a chance, I'd maybe bring it up again and say something like 'look, I know you have a lot going on, but if I can repeatedly feel that and don't get what I need from you, we have to somehow resolve this'.