Simple technique for preventing bugs like this: don't copy-paste code. If you find yourself copy-pasting code think twice why you even have to do it (DRY principle) and be aware of the potential consequences. Even if some code has to be duplicate, I am forcing myself to just write it from scratch, exactly for this reason: do you really know that you have updated all your data? And yeah, unit tests would help in spotting this and other things. But aren't unit tests a duplication of your code, already?
UPDATE:
Maybe I should have stated my last question differently. I meant that in the context of checking that the data is correct, it would be the same as writing the duplicate code from scratch.
UPDATE:
Maybe I should have stated my last question differently. I meant that in the context of checking that the data is correct, it would be the same as writing the duplicate code from scratch.