Here's something that's been goin on a very long time, and is very frustrating.
The reason I havn't reported it, is because it doesn't happen every time, so I tend to let it slip my memory.
Sometimes, after typing a post, and then hitting reply, I get this :
"This page is asking you to confirm that you want to leave - data you have entered may not be saved.
(and these 2 options) Stay on Page ? Leave Page ?"
It's odd because it's the same kind of message you get as if you started to type a reply, but decide not to submit it, but instead close the browser window, or click somewhere outside the thread, without submitting the message.
However, that error message is occurring after just a simple, routine message submission.
And at first, it becomes confusing, because neither option sounds like it will submit the post.
I think if you choose, "Leave page" it submits the post, ironically, even though it confusingly sounds like it will leave without posting it, especially with the comment "data you have entered may not be saved".