Reporting for Punishment...
Hi, I was the culprit DM....
My error was based on a long-held belief in our group (inexplicable as it turns out) that a character could pass through an ally's space freely *unless* that character was in combat.
That's the way we've always played it.
I wasn't attempting to kill the character at all, just playing by (my interpretation of) the rules - I thought he could take the extra hit in any case, but I'm not one to pull punches...
Another error I made was in not permitting the medium sized Ftr to pass through the squares occupied by gargantuan and colossal centipedes (Lucifus, Lucifus, tsk, tsk, tsk... the party looks ever more nervous with each passing summoning).
As for the PS - I was treating that as a large object rather than an ally. Earlier in the prior session a group of panicky peasants tried to mount and ride off on the PSs that the party had left parked in the fields outside a plagued village. Unfortunately they were getting nowhere due to the fact that the steeds hadn't been conjured specifically for any of them, and they ran off in terror when one of them noticed that the horses had no feet.
My concept of the PS was of a solid mule-like critter that couldn't be budged by anyone except the "owner". Hence, in the Ftr vs GSG case, I allowed the Ftr a bullrush attempt, which he failed.
So all in all, three errors, which I chalk up to experience.
It's a sad tale, but one that has a happy ending, as I'm sure Lucifus' story will relate.
For the record, the GSG was not destroyed, but the group (actually the Ftr!) figured out how to placate and bypass the ornery lumpkin.
Any they all adventured happily ever after....