The Fighter Vanguard's TAB power, Dig In, is now getting cancelled when you run out of stamina. If you then press TAB again without any stamina, you can continue to use Dig In.
To replicate the bug:
1) Enter into a fight with mobs or a boss, and use Dig In.
2) Allow them to reduce your stamina to zero.
3) Your character will then stop using Dig In.
4) If you use Dig In again, you will be able to keep using it.