It's rude to flake out on plans.
Sammy had planned to stop by her friend's house after work but learns about an opportunity to do a project after hours which would look good at her job.
Sammy wants to improve her chances to get a promotion.
Sammy stops by her friend's house and stays for a short time before going to work on the project.
Sammy lets her friend know she can't make it and takes the opportunity right away.
Sammy's friend is happy that she was able to hang out for a bit.
Sammy's friend feels like it is unreliable to make plans with her.