Using an unsigned variable in a computer program can lead to unexpected errors which we call overflow errors.
This where if we subtract one from the variable when it is 0 it becomes an unexpectedly large number like 4294967295.
This is a /wiki/spaces/IX/pages/2053242952which is relatively harmless although alarming looking.
But it’s also the same problem that caused the French Ariane rocket to explode (a $500 million dollar disaster - see https://www.bbc.com/future/article/20150505-the-numbers-that-lead-to-disaster).
Google around and you’ll find all sorts of expensive problems that arise from this issue.
How does it happen? Read about how computers handle subtraction.