I made changes to an SQL view used as the source for a pretty important integration that is built and managed by an external company. The integration failed the morning after and I was immediately blamed by the company and heard "how could you be so careless" and "how long have you worked in this business". I've been a programmer for 20+ years and done integrations for 15+ years. I know I checked the output of the view and it was identical in every way to before my changes.

After finally getting access to the integration code on "the other side" I found that it didn't read from the SQL view - it read the view definition. It also uncommented anything in the view (yes, uncommented) and ran that query.
We now have a year free of charge - which we won't need because my boss is throwing the company out as soon as we have a replacement.