a few years ago a colleague of mine literally begged me to cover her double shift (14 hours) and in return would cover one of mine later (common in shift work) I don't mind because I got paid for covering her shift anyway. BUT a couple weeks later I ask her to cover my Sunday morning shift so I could attend a friends party on Saturday night and she agreed, then literally 10 minutes before the shift on Sunday she messages me saying she can't cover for me because her friends invited her out last minute and she's hungover????? I told her it's unacceptable and she told me she doesn't care because it's my shift so it's not her problem.