I work in a hospital laboratory, we are staffed 24/7 so various shifts need covering. We've had a lot of short-notice sickness due to COVID and people needing to isolate. On the whole work has been very good with this, as they should be. But where a staff member gets signed off, their shifts need covering, sometimes with as little as a day's notice. I had been picking up a lot of those shifts, as I could do with the overtime money, but they became reliant on me, as they knew I'd want these shifts. I did a two week stint, covering a lot of extra shifts and nights, and due to the work pattern I was on and needed to be changed into, I ended up owing them hours because I changed my hours to help cover the gaps in their rota. For example a double nights shift needed to be covered, so I had to swap 4 days for 2 nights (due to mandated rest). This pissed me off, and I let them know it. I refused to sign up for any more overtime and they could cover the shifts themselves. It took the senior covering just one of these shifts for a change in policy. We're now given a short notice enhancement if we sign up to a shift in the coming 7 days, and the big winner here is no hours need to be making up. I can be rota'd for 4 X 7.5 hour days, swap into 2 X 11 hour nights that need covering and not have to make up the missed time. Which is a great deal, for both sides; shifts that need covering get covered, and I don't mind shifting my life around to help out my colleagues because I'm not getting screwed on my hours.
TL;DR: was getting screwed over by covering staff shortages, refused to do anymore, manager covered one shift and has now negotiated better benefits to covering these shifts.