When you connect to a hosted server and are missing a mod, you get a download prompt. If you press enter there (which can easily happen if you were just about to chat), the mod download will be rejected. This is non-intuitive and requires you to reconnect to get a new prompt. In the meantime all other players keep receiving a repeated hint that you are missing this mod.
I think when you hit enter there, either the download prompt should be accepted, or, probably maybe better, this should have no effect on this prompt. Hitting escape, however, should allow to bail out from the prompt, denying the download prompt.