I heard about this from a scripting helpers thread, and can confirm that it also happens on my machine.
This bug happens when StarterGui:SetCoreGuiEnabled is used to disable the chat GUI. Pressing the backslash “/” key will halt character control. If you were walking as you press the backslash key you will continue to do so, even after you let go of the walk keys. Neither pressing the escape key nor clicking anywhere on the screen will re-obtain control to your character. Resetting will.
This bug can be replicated by using game.StarterGui:SetCoreGuiEnabled(“Chat”, false) in your game.