My worst experiences (from the top of my head):
Project participates in Hacktoberfest but doesn't review nor label-acknowledge PRs.
Project takes PR commits and replaces author when "merging" into main.
Invest a day into a 16 months open thing, create the PR, maintainer a few hours later: "I just implemented this". Probably an unfortunate coincidence, but feels like sabotage.
Contributing to GitLab, even simple docs, became inaccessible, infeasible. When I see typos or link markup mistakes in docs I create a PR. GitLab has a whole, elaborate onboarding process with guides, ticket creation, talking about yourself, waiting for approval, etc. Then you try to edit and by necessity fork the GitLab repo, but it reaches free account repo limits which produces unclear, confusing, inaccessible issues while trying to make and submit the changes. So you learn about the separate reduced repo you can actually fork and work and submit on. So you do that. Elaborate PR description template, which I remove, because it's a simple typo/syntax fix. They ask for more or sth. I quit; I close the MR and leave.