Steps to reproduce:
Your HUD has been locked.
You go to an area, where it automatically becomes unlocked (terminal, subtown, Haven, ...).
People say stuff until the chat area fills up and becomes scrollable.
You enter an area, where the HUD is returned to locked state automatically (clockwork level, training hall, ...).
Effect:
The scroll up button of the chat remains click-able, even though the rest of the HUD is locked as expected.
The HUD is now in a kind of semi-locked state.
When you try to walk around using the primary mouse button (or do whatever your primary mouse button has been bound to) and accidentally click on the chat scroll button(s) nothing will happen, except for the chat scrolling, leaving you exposed in some combat situations.
Annoying workaround:
Unlock and re-lock HUD.
Additional note:
I am not sure, but this bug may also occur in other situations, when the HUD is locked and someone of the party says or emotes something, while the chat is not yet filled up enough to be scrollable, but becomes scrollable at this moment.
(EDIT)
It appears to me, this not just happens with player messages, but also with all kind of other messages, starting when the HUD became scrollable and reoccurring on every new message, even after the above mentioned workaround has been used.
The small + or - in a circle, that toggles the Current Objective message is also unaffected by locking the HUD. Unfortunately it appears in very danger areas (e.g. Firestorm Citadel), where clicking it accidentally, instead of moving, can cause instant death.