You would be surprised how easy it is to hit that extra zero on the newer cash registers, so when entering $10, you may get $100 instead. BUT, there is no excuse for not being able to make the correct change. I have done it, and laughingly said to the customers, Sorry about the fumble fingers...as much as I like you, I can't give you the change for the $100, but I CAN MAKE THE CORRECT CHANGE, and did it. People enjoy seeing that someone can still do it, even if they can't "type" properly on the register, lol.
It is such a simple skill, no kid should get out of grade school without being able to make change, or count backwards.