For me it was writing a scene where a stolen dog is taken from her puppies and mistreated. Gack, it had to be bad enough that you hate the villain, but not so bad that the reader can't read on.I feel for Mr Frost.
When I killed off my hero, I cried on and off for two days. And again when I went back to edit it.
I was walking round the supermarket in dark glasses in January in case the neighbours thought something awful had happened.