Comment by extraduder_ire

The commit you're amending doesn't exist on your copy anymore, despite existing on the remote that you've pushed to.

Warnings about detaching the HEAD in git are so common that they're kind of assumed to be basic knowledge to a lot of people. Explaining why it's bad if you don't already know would take quite a while too.

I do wish git had some form of warning before you rewrite history that's been pushed somewhere. It should already have all the data to know that.