After you've agreed on a plan, your friend suddenly says they can't make it anymore and asks to reschedule. Respond and propose a new plan.
Your friend suggests a time that conflicts with something else you have. Politely explain and negotiate a different time.
After agreeing on the activity, your friend asks, "Actually, should we invite [someone else] too?" — react naturally and decide together.
You both think you agreed on a different meeting place. Sort out the confusion and confirm a final location.