It's when the other party denies it, gives excuses, acts like they don't care, etc. that people get pissy. That's when IP owners get notified, and angry blog posts are hammered out like Stephen J. Cannell at his typewriter. All too often, it turns into a complete clusterfuck because the person at the center of attention gets hot under the collar and somehow screws up. They might say something they regret, reveal how ill-informed they are about the situation at hand, or otherwise get people more riled up by doing things like threatening defamation lawsuits or claiming there is a conspiracy to steal their ideas. But all it does is getting everyone else even more pissed off.
I'm so mad I popped my collar
Sidebar: no shit regarding conspiracy claims. This has happened twice now in two different incidents. Trust me, if your stuff was that good, WotC/Onyx Path/whoever would hire you or buy it from you. They wouldn't steal it. Yes, that means your shit isn't even good enough to steal.
To the Lakims and Alleys of the world, there is only one solution and it's the same one that has been repeated over and over: OWN YOUR SHIT. If you fucked up and accidentally reproduced an entire chapter from someone's fanfic and there are people right there who know what they are talking about just fucking own up to it, fix it, move on and don't do it again. No excuse. No explanations. No arguments. No "But it's perfectly leeeeegallllll". Don't put "ego’s sociopathic cousin, hubris" (as Patton Oswalt put it) in charge. You're not going to win, ever.
Not because there are Internet mobs and we just have to live with it or because you're a horrible person. Just because you need to own your shit.