angry emails and api breakage
I did some overhauling to a module a few months ago. I added a new interface to it, and left the old one in with a deprecation warning. After a little while, the original author uploaded a new release that totally removed the old interface. Within a day or two I’d received comments on IRC and via email. They were all something like, “The new interface is definitely better, but I had code using the old one!”