For a real world example of cut-n-paste wrong doing: This idiot had made multiple modules with this error in it: The code was Info-Basic, but I'll present it "Perl" style

for ($x=1; $x < 999; $x++) {
 ProcessAccountNumber($x);
}
[download]

He did this for every single number based item. ("Gee, we'll never see numbers that big.") Yes 999, for transactions he used 999999.

Ow. remembering that hurt.