Essentially, you can’t think an acorn is a bullet and get away with shooting at a detained and secured civilian. But, if another officer on scene thinks, even unreasonably so, that an acorn is a bullet and starts shooting at a detained and secured civilian, you can too. If this doesn’t make a lot of sense to you, take that as reassurance that your critical thinking remains, at least partially, intact.
IIRC Sympathetic Fire seems to be insta-forgiveness (by other police and the courts) whenever it comes up.
As one example, I think it played a role in the Daniel Shaver case, but it’s been a long time since I read all those details and I really don’t want to dive into that pool of anger and sadness again to verify.