This happened to me earlier, and I don't see how I could have hit the merge button without meaning to. I'm not sure if I can replicate, but I thought I would file it. I hear @puiterwijk may have hit this once.

I had filed the PR shortly before.

https://pagure.io/koji/pull-request/565

I started to write a comment, then got distracted for a while. Later I came back and clicked the button to submit the comment. I was directed to a page saying something like "waiting for your task to finish". When I reloaded the PR, my comment was not there, but the PR had been merged.