This is not my proudest moment because I knew better: Once I screwed up the Google ranking of a client site.
We were doing a semi-major overhaul of the site and shortly after going into production, the ranking of the site tanked. And why? We had forgotten to redirect the old URLs to the new.
Not only did I actually know this – it just wasn’t present in my mind when we worked on the project – but we could relatively easily have fixed the problem as we had the necessary data to reconstruct the old URLs and which they should redirect to. Unfortunately I was unable to communicate this to the client, who insisted that the work had to be done by his SEO people.
They obviously didn’t have the actual information so they had to reconstruct what they could from Google and then tell us what needed to be done.
I am unsure if the site ever recovered its ranking completely.
Back then, and it is a long time ago, I realised that this must happen all the time and yet I have met very few people who are actually prepared for it. It is a simple task to do before you launch the new version; if you don’t it’s a very hard task afterwards.
Henning's blog 