I guess if someone cancelled a significant percentage of the time because their kid was grounded, I could see labeling them flaky. To me, flaky friends are the ones who regularly call you 20 minutes after you're at the destination to breezily say, "Oh hi! We just woke up, so we'll be there in like an hour" with no apology. Friends who are typically punctual and dependable, but who very occasionally (like, 1 or 2 times a year maybe, if that?) call and say, "I'm so sorry, but we've had a really rough morning with DS's behavior and I've told him he can't go to the movies today" don't qualify as flaky to me.
Thanks. I was coming in to say something very similar.
My son is very social creature. No amount of talking, distractions, time-outs or taking away a favorite toy ever helped him when things got tough. But break a playdate? Whoa, mama. I only had to do this a handful of times but it really hit home with him.
My daughter could shape up with a stern look.