Sometimes it happens. I'll say I want to see this movie, and then he'll watch it with buddies. Idk, it's not a huge deal to me, because it doesn't take away from my ability to do it with him. It would be different if we actually set a date, and then he "backed out" of it and instead went with friends... like if we made dinner reservations, he claimed he didn't want to go anymore, and then I found out he actually went with somebody else? That's not so OK to me but that's lying.
The only thing that I can think of that happens like this is plans to go to the gym. I love working out with him (because of the post-workout fun...) but usually he'll be like, "Oh yeah, let's go this friday" and then Friday rolls around and instead he is going to go climbing with his friends or something else. It's kinda meh because I know he likes climbing and it's an activity he's always done with his buddies.
Usually when you make tentative plans like, "Oh we should do this sometime...' but don't make any PLAN-plans, your SO probably isn't going to think "must save this for a date alone." If it gets brought up again with somebody else and they make real PLAN-plans (like time and date specific plans) they're gonna go for it.