Don't know if this can be replicated with a keyboard and mouse, I was using a gamepad at the time. PC Version of course.

While mining I opened up the menu and went to my friends list to see if a friend was online. After backing out of the Friends list to go back to mining, the mining menu was gone and I could not exit the mining stance. I could not log out or exit the game, as the game stated, 'You cannot perform that action as you are currently mining'.

I eventually managed to get out of it by removing the prospect ability, using the back button to select the buffs bar which let me out of the mining stance and allowed me to carry on playing.

Do not know if this is easily replicatable as I never tested it. Other ways may exist to break out of the bug, like opening the menu and selecting the gathering log or something. Opening any menu may break it back out.