Early in my career, in the newspaper industry, a new system was developed that focused on new features/capabilities and ignored some existing features and the response time of the old system. The switch to the new system was done as a cutoff, without giving the people involved much detail, even though it was a drastic change. The new system had more features, but people were used to the old system so the new one slowed them down, and speed was essential for them to do their jobs well. Finally, after proper fixes and staff training, it went OK, but the development team suffered for a couple weeks after deployment.
I learned not to jump to production until there’s been a test phase or a lot of training. I was junior at the time, but the lesson stayed with me – for every project, I ask “OK, did we cover that? Are we missing something that’s really key?” Sometimes if you miss a detail it can be a nightmare when you go to production.