Had someone inform me this morning that they posted a comment to a post of mine overnight and it didn't appear. According to them, they'd signed in with their WordPress account, typed a comment, hit post, and... nothing. It's definitely not posted, not in moderation, and not in spam.
I can reproduce the problem, and determined why, sort of. Upon hitting post, the Jetpack comments iframe refreshes to... a page that's a 404 error.
It goes to
which doesn't work, because it should be wordpress/wp-comments-post.php?for=jetpack
If I disable Jetpack comments, native WordPress commenting functions fine. So, it's something about Jetpack comments being active.
None of the other various threads about this problem -- which seems to crop up every few months? -- leads to a solution for me. I've no idea how long the problem has been around; the last comment I have on the site is from January 2, but I have no idea if anyone's tried to post one between then and today.