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.
I have the heuristic that I leave decisions for the end of the day, based on what I have done. In fact, this is one of my goals per day, when things are unclear.
The habit of skipping them for whatever reason is also problematic. If you so easily skip them (as a team or as individual(s)), why have them in the first place?
Greece. I learned a lot in the process having just an undergraduate degree at the time. The students didn't appreciate it much unfortunately (wasn't a university but a vocational school; don't ask why Tanenbaum was on the curriculum). Just one came to me after the exam (I had to pass them all anyway) and congratulated me of making it so hard for them (him).
I remember writing an essay like this for an assignment. The professor called it 'strange'. Even if I had made some valid points, there was no way to validate them as it lacked real flow. Just bullet points with messages I wanted to communicate but with no real value on their own (and some symbolic paragraphs in between). Good to see a post calling out the naked emperor. The same post had received a better reaction in the comments the last time.