I don't know if this is the genesis, but the first time this came to my attention was with Linux. Basically: a third-party trademarked Linux several years after

I don't know if this is the genesis, but the first time this came to my attention was with Linux. Basically: a third-party trademarked Linux several years after the project's origin, then started going after companies that were distributing Linux. Legal battle ensued. Linus Torvalds was eventually assigned the trademark. Innocence lost. Trademarks likely existed for (what we would now call) open source software prior to that, but I think Linux was a turning point since it meant that community (rather than commercial) software now had to consider such prospects.