I work part time in a small company, we're minimally staffed with only 2 people on each shift.
Recently 3 of full time team members have left but our employer is only willing to hire one full time and one part time team member to replace them, stating that since there will be 4 part timers we can pick up the extra shifts. This means that if someone is on holiday and someone else is off sick on the same day that one of these 'unstaffed' shifts needs covering then either 3 of the 4 part timers will need to make themselves available or a full timer will need to work 6 days with 2 part timers on.
I made it clear in my interview that doing more than the occasional extra shift would be difficult as I only have childcare for the days I'm contracted to work (friend one day (I look after her dc on one of my days off in return)/PIL the other). In fact all of the part timers have other commitments that will make picking up between 7-8 extra shifts a week (not including holiday cover or sick cover) difficult, it's why we chose part time work.
Employer won't listen though and fully expects all these shifts to be covered. If they aren't then it puts staff at risk as we end up with 1 person opening/closing when, for safety reasons, we're meant to have 2 :-(