Tipping Error - Should it be Fixed?

i m h o... the error should be corrected. i can just hear the server conversation at the register... 'the sever says " i just got $60.00 on $300.00" the envious other server looks at the charge and says " well, they wrote $380.00 as the total, they must have meant to tip you $80.00". server one says "you know, i was awesome to them, they must have meant to tip me $80.00". server one pays out the charge and walks with the extra $20.00. my servers are always asking me questions about everything, but mysteriously become silent when poor math is in their favor...l.o.l. they are regular einsteins when the check total and bottom line total are against them.
as an aside, i have caught more than one server over the years who took the liberty to add a tip to a charge when the customer accidentally took both vouchers. only to be busted by the customer who calls back the next day to say " i am so sorry, i took both copies, can you add a tip???"

Most restaurants have a policy about this, and the only one I've ever agreed with is:

Enter the tip they write in the tip line. What if the customer mistakenly wrote $260 instead of $360 for the total? That server does not pay the difference for the mistake. Computers do the math for the total. I've always thought the bottom was for the customer's record - this has always been a no-brainer for me.