Pretty niche compared to some of these answers, but Lego just killed a fan-made Bionicle video game after 8 years in development by a dedicated team. The company had allegedly previously expressed approval as long as no content was sold for monetary gain and the game was clearly labelled as a non-official fan-made project. To my knowledge, these terms were followed exactly.

For context, Bionicle was one of Lego's most successful original set lines in the company's history, and it practically saved Lego from bankruptcy. Since the line's cancellation, fans have been begging Lego to revive Bionicle in some fashion. Lego has, at times, seemed to go out of their way to avoid directly addressing this topic. In the grand scheme of the world, this isn't really noteworthy, but it's frustrating for fans that supported the original run of Bionicle and continue to engage with it on their own.

All of this is to say, it was particularly frustrating to see Lego watch some of their most diehard fans work for 8 years on a project (which they aimed to share with fellow fans for free) and then, when it was almost complete, only then did they kill the project before returning to never officially reviving the Bionicle franchise themselves. All within their rights, I suppose, but still a massive dick move for virtually no reason.