I haven't been able to pinpoint exactly why it happens, but occasionally, the chat log suddenly manages to skip a line, and stops scrolling with each new message. If the HUD is locked, the arrows are all greyed out, so you get no clue that it has happened until you or someone else says something, and it fails to appear in the chat log - at which point you realize you've missed out on quite a few messages.

I think I've mostly noticed it happening while going down the lift to the next floor.