If a user’s mouse has been locked by the MouseBehavior property, and a GuiButton’s Modal property has unlocked the mouse, the Delta of the mouse will still be recorded, even though it shouldn’t be doing so.
This issue can be seen very clearly in a game like Apocalypse Rising:
In case is isn’t clear as to whats going on here, I’m in first person, but my mouse has been unlocked by the inventory UI. However, the mouse is still recording the delta as if it was locked, so the camera is rotating as I move my mouse around the screen.