If you are typing in chat, and you press Enter at the end of a sentence when your characters haven't appeared in the text box yet, it sends a partial message.
That has to do with how characters are processed. Any game affected by lag will have that effect, ergo, you're lagging For a similar example, here's Microsoft Support's take on why such things may happen in broweers [and you can assume a similar mechanism at work for in-game character lag]: tl;dr, you're sending a 'complete' command before the 'write' command finishes, and it's interrupting the process.
I tweaked the phrasing to be more clear, is it understandable now? I was concerned I wouldn't be able to adequately describe how chat processing interruptions work, so I referenced something that referred to the same premise. I assumed it would make it easier to understand. :X The tl;dr at the end was the only point of note.
