Achilles Heel

Achilles, the hero of the tragic Greek myth of the same name, was invincible because his mother immersed him in the River Styx when he was a child. To do this she had to get a firm grip and push him under. She happened to grip him on his heel, which was therefore not exposed to the water. Achilles was eventually killed by an arrow which struck his only weak spot - his TragicFlaw - his AchillesHeel.

In WorldWarII, for example, the British battleship Hood was sunk, during the effort to sink the German battleship Bismark, by a single shell that penetrated minimal armorplate at the base of its stack and exploded in the Hood's ammunition magazine. This small chink in the armor of an otherwise well-protected vessel was the Hood's AchillesHeel.

In modern system-speak, we also refer to this as a SinglePointOfFailure -- but I prefer AchillesHeel.

I don't think they're the same thing at all. A SinglePointOfFailure is something that isn't redundant - and if the thing fails, the whole is disabled. It says nothing about the protection of that thing.

And the Bismarck's rudder was jammed, dooming the ship, in a torpedo attack mounted by airplanes that were so outdated that they flew slower than the Bismarck's anti-aircraft fire control had a setting for. An odd AchillesHeel indeed.