When you are cloaked with minor cloak as a Heavy, or Medic, and you try and shoot your gun, your ability gets activated instead of your gun firing. The only way to stop this is to move quite far to completely drop out of cloak and then you can shoot your gun.
Found this bug also applies when you ride jump seat on a cloaking flash. When you try to shoot your gun, while cloaked, your ability activates.
