Where I work they plan what overtime shifts need covering. If you say you will do it and then fail to show, you have taken an overtime shift that someone else could have had, and you let the company down. We have a policy where if a person fails to show for overtime they cannot do overtime for a certain period, that period increases with the number of times someone fails to show up.
If you have a valid reason for not showing up (we all have emergencies) then phone in as soon as you know you cannot attend, so alternative cover can be sourced if required.