IE11 is not IE8, guys.
This. It's tiring to see that a legitimate bug is being reported and all people can do is make smart comments about a browser they're too insular to realise is actually the fastest out there right now.
ON TOPIC: This has happened with me on Chrome before. From what I can tell, for some reason the there's always a "space" already entered in the reply box by default. This makes the page think you're writing something, thus prompting the message.
One day this randomly stopped happening to me so I can't tell you what steps I took to rectify it. I would check to see if this is what's causing it for you, just so you're certain.