Slop.

What I mentioned is not an exhaustive list, but in terms of specific political movements, those are the two stories that I see most often, the "betrayal" of the Spanish anarchists (and Trots) who thought that a civil war with fascists was a good time for a revolution and therefore sabotaged their liberal allies (the Spanish Republicans, who the Communists were working with), and the betrayal of the Makhnovists who had genuinely been engaging in banditry on Bolshevik-controlled cities because, despite being very militarily effective, they (the Makhnovists) were not economically self-sufficient or really productive at all, and that's before you factor in having maybe their whimsical monetary policy, because the root issue had more to do with relations of production.

I'm not going to pretend to you that anarchists have never been wronged by communists, whether in Spain, Russia, or elsewhere, but the specific examples that I usually see are instances where it would be more accurate to call the anarchists the "traitors," but the communist retaliation is framed as the first shot. I am quite confident that you can go through the history of the Red Terror, of the post-Cultural Revolution crackdowns (and I mean under both Mao and Deng) and find much more genuine misdeeds, I'm just less familiar with these because they aren't thrown in front of my face all the time like the stories I mentioned.

It should also be mentioned that even though I think Mao ended his career with some of the gravest betrayals of a political project that I have ever heard of, he and the CPC had facilitated various anarchist and anarchist-like projects from the interwar period until the unofficial end of the Cultural Revolution, and there wasn't zero communist interference, but generally they had neutral or supportive relationships (with some communist factions being much more hostile, one going as far as assassinating a KPAM leader) until various factors (Japanese aggression, poor construction, etc.) caused them to fail. That and Mao's crackdown ending the CR was mainly to stop country-wide gang violence, meanwhile the more substantial anarchist-like projects like the Shanghai People's Commune were dissolved in a more orderly fashion as they didn't perform as well as hoped and were deemed effectively to be left-deviationist.

Anyway, I wouldn't ascribe a "level of historical knowledge" to myself, I just know some stories, and the accusations led me to read a bunch of different anarchist (and Trot) accounts until eventually I found bitter anarchists who nonetheless admitted to things like the Spanish anarchists sabotaging the Republicans. I encourage you to look up things on your own and treat what I have to say like a Wikipedia article, as mainly being a basis for further research at most. If someone would like to offer corrections, I am happy to hear them.

My real thesis is that these myths of aggrievement are just the Red Scare as processed by, well, another group of people who don't seem to have more nuance in their accounts than the neoliberals and are therefore happy to have the same boogeymen following roughly the same logic. Besides the blatant revisionism, I'd respect it a lot more if the people in question were just more upfront about the fact that the substantial divide (where there is one, and that really depends on the anarchist) is about democracy vs autonomy, where the communists demand the continuous advancement of the former and anarchists the latter, and these two things are inevitably at odds.