I believe it was one of the UFO:After... games by Altar. But think of the health bar as fully green. You shoot it with a stunning weapon, it takes off a small bit of health, but adds grey, so your now @ say 50% grey, 40green 10red. 90% total health, but 50% stun. You then shoot with the lethal, taking off 45% health. The stun amount doesn't change. But now total health is 45%. And the bar is now solely gray and red. And that's how you brought the current health below the stun level, causing the stun.