the worst is when there is a thing you particularly want to do, in a particular place, so you say to your friend "Let's go to that café / see that film / go to that antique shop / go to that orgy / whatever". and your friend says "yes" and you think "yay, next Saturday I get to do a cool thing with my lovely friend!"
Lovely friend invites person y. you're kind of nodding along when they break the news about this. It doesn't make that much difference - it's just a movie / café / orgy, anyone can go, lovely friend is just being lovely and friendly, sure I can deal with it (even though person y is boring and tells long stories about themselves with the inevitable punchine "and that's why I was right", and otherwise echoes things you have just said as if they have just thought of them and you are supposed to laugh as if it is their joke, becayse they look around for applause, even you you just said it. But never mind, live and let live, etc etc etc)
and then person y goes "well, I can't really make 11, let's make it 2pm. And I've always wanted to go to a carboot sale that specialises in rabbit hutches, so let's go to that instead. And I don't fancy that café, there is a burger van at the carboot sale which does great burgers made of fleas. GREAT! CAN'T WAIT!"
And you are supposed to go along with it.