I'm currently on my final warning from my GP after missing two appointments, so I just don't go the doctor any more.
I've missed exams.
I've missed events which are very, very important to me.
I've missed deadlines to claim thousands of pounds worth of money.
I've missed flights, but I have found a way of avoiding this, which is to book an early morning flight, and arrive at the airport the night before and try and get a bit of sleep on the floor near the check in desk. I still panic about it, though, and spend and hours obsessively going over contingencies.
Most of those things happened a lot more before the ADHD diagnosis, when I was trying to do things the neurotypical way, and thought that if I just tried a bit harder, I could be on time for things.
I can usually manage to be on time for a regularly repeating event, so if I were to have a standing date for brunch with you at the same café every Saturday at 11, I would probably be mostly on time after the first 6 weeks or so, once the preparation had become a routine, rather than a vast sequence of decisions which all have to be made correctly in the right order. And the hours of planning that go into developing a successful, timed plan for leaving the house and going somewhere are worth it for a regular event, but I can't put that amount of effort into every meet-up I have. .
And if I'm late, I might make a joke about it, because that's a lot more socially acceptable than being honest about the immense shame and guilt and sense of failure and worthlessness I carry around with me because things that other people can do with just a bit of thought and effort feel like the labours of Hercules to me.