People can forget, change their mind, etc. or, what I do, wait for further information from the person who invited me. I don't count on things happening, even when I set them up, until the plans have firmed up and everyone has "seriously" agreed. Just saying "You want to go to the Concert this Saturday night?" and having them say, "Yeah, sure!" that's not plans yet for me. If someone invites me and then doesn't say anything further, doesn't get in touch with me and firm things up, I figure they've changed their minds or decided to do something else or invite someone else or whatever.