I have had two major experiences with plagiarism/theft and both of them were completely and utterly infuriating.
In one case, when I was playing on a WoD mage game many moons ago, someone stole my entire wiki page, swapped out maybe three or four sentences on the whole page, and plunked in (some) new pictures. We’re talking the CSS, the color, the layout, the writing, everything – including some images that were icons for specific subsplats that this new person didn’t belong to but didn’t recognize and was too stupid to remove. The amazing thing was that when I confronted her about this and pointed out that her wiki page history showed that, when created, it still had my character’s own name in the content, first she a) denied that she had taken any of my work and then b) insisted this was fine because the site wiki had templates. Which it did. But my writing, my graphics, and my custom code are not. So theft, and then outright lying to my face.
I experienced something similar when someone I’d brought on to staff BSG: Orion after Dropkick left lifted our entire database to help one of the former staffers build BSG: Deimos. Now, as it was explained to me when I started on that game, the BSG games had a tradition of passing on a lot of work from one game runner to the next, so most of that was totally fine. But I was also under the impression that they did so after talking to the previous staff and did not do things like port in room descriptions, unique mechanics that hadn’t existed on other iterations, etc. – all of which BSG: Deimos did. And when I confronted one of their staffers about it, they once again claimed that they hadn’t taken anything, all of this was passed from game to game to game, and which I knew for a fact wasn’t true because I was the one who wrote some of those things, as well as several other new things that had been created by staffers that weren’t around, weren’t consulted about reuse, and blatantly had their work taken. That really upset me because I felt like not only had my work been taken, but that I had let them down by trusting the coder I’d brought on.
In both cases, the thing I found to be beyond the pale wasn’t even the theft. It was the blatantly lying to my face about it. If you’re going to be an unrepentant asshole about stealing other people’s work, at least own it. Own something that is actually yours, even if it’s just your shitty actions and attitudes.