every now and then you might copy some code from here and paste it there forgetting to update to pasted code and the worst thing is that it might still work :).
then you get to know about this in production. worst when someone else pushes a patch titled copy paste error and adds you as the reviewer, have been there myself, not a good place to be in.
if you have such copy-paste errors in your code, that means that you are not reading the code properly. one should first go through the code token by token and only as they parse the token in their head, they should think of it's high level meaning and where it fits in the mental model.
while parsing each token, ask yourself, is this the right token? does this belong here? is it used correctly? and based on that decide whether a token should be there or should be removed. this should be done while writing the code, while copy-pasting code and while code reviewing.