I was looking at the Terminator entries in Codex Space Marines last night and noticed something very odd:
It appears that Terminator squads don't get a 5+ invulnerable save.
Bear with me. Nowhere does it say that they have Terminator armour. The 2+ is in their profile. The +1A is in their profile. Deep striking is listed as a special rule. No mention of a 5+ invulnerable.
Now, I haven't noticed this before and have been playing that they do have a 5+ save since it was introduced in third edition. I also believe (but obviously can't prove) that the intent is to give them the invulnerable save.
However, how can anyone justify using such an advantageous rule when it isn't listed in the codex?. "Oh, so you want a 5+ inv save on your Terminators. Fine, all my grots are monstrous creatures with lascannons and lightning claws."