Ouch!
I've made errors that big in my career - but they've always been weeded out through processes put in place for just that purpose.

No one needs to get fired over this but you need to slow down and establish procedures that help eliminate the mistakes that everyone is capable of making.

Redundancy is important. One person needs to be making the same changes on an independent spreadsheet. One stays on a local machine and the other gets passed around for changes. When the two stop matching, you know there is an error.