[starbug]

i have discovered a rather odd bug in fred, i am using the latest build. The bug is that one ship i place in a mission will become invulnerable! Even if i don't use the invulnerable sexp or the invulnerable option in the ship editor( i triple checked the mission just in case). Also its never the same ship, one the same mission it will be say a Mara then when i replay the mission it will be a Orion thats invulnerable  i am not using any mods, i have the latest builds and the 3.6.8 medias. Any clues?