Ask HN: What are your good, bad, and ugly refactoring experiences?

It can be refactoring, retooling, upgrading to a newer version of a framework, adding tests, etc. There are times it goes well, and there are times it goes poorly.

Could you tell me about your best and/or worst refactoring & retooling experiences?

Details I'd really love to know:

  - Why did you choose to refactor? (testability, personal desire, preparation for upcoming features?)
  - Who did you talk to about the refactor? (other engineers, product, team leads?)
  - When did you talk to them about it?
  - How long did you expect it to take?
  - How long did it take?
  - What went well?
  - What went poorly?
  - What did you learn?

Context: I'm researching to write a blog post about good processes for refactoring on a software team. I'd also love to hear from PMs & non-engineers about their experiences with their teams doing a refactor.