1)you are typing a "new" message, that is the only message since the last dotted divider
2) the message is an action (/me)
This is fairly reproducible, at least in the last few days
As you can see, on an Action, the skin doesn't increase the padding, I guess? sufficiently to make room for a user icon as it does when one types a regular line of text.
Or maybe it's related to the fact that actions occur on the same line, whereas regular text gets pushed to the next line?
Anyways, not a high profile thing, but I thought I'd bring it to the style dev's attention.
(Interestingly, when I asked a friend to try this to see if it affected only myself or both myself and remote users, the behavior was to display the line correctly, initially, but when I would type another line, it then displayed the dotted divider line through my friend's userpic, and remains that way until scrolled out of buffer. I have a screenshot of that as well, if it's necessary
)