In our Felltower game on Sunday, Aldwyn went to more than -1xHP and had to make a "death check." He rolled a 17. As usual - his last one that he rolled was an 18, the previous session.
I ruled he was dead, given the rules discussed here. The player pointed out he had HT 13, and Hard to Kill 2, for a 15. So a 17 is a miss by 2. A miss by 1 or 2 is a Mortal Wound, not instant death, per Basic Set p. 423 and Exploits p. 60. His guy had just died the previous session, so I said okay, he's Mortally Wounded.
In the future, though, I'm ruling differently - and I feel as the rules intend.
Per Basic Set p. 348, and Exploits p. 7, "A roll of 17 is a critical failure if your effective skill is 15 or less; otherwise, it is an ordinary failure." I feel that rule for Mortal Wounds doesn't trump the basic rules of success and failure. A character with HT 16 who rolls a 17 would be mortally wounded, and one who rolls an 18 should be dead even though it's a failure by 1-2.
A critical failure should be bad, even if the rules provide for a failure margin that has a lesser effect. Playing at the top end of the bell curve shouldn't forgive you that except as specifically noted (i.e. 17 becoming an ordinary failure.) Even though specific rules generally trump general rules, I feel this general rule should carry through. Otherwise it's possible to be practically immune to death - death only 1 in 216 times (and with Luck, make it possible to die outright only 1 in 10,077,696 times.)
All of that said, for the future sessions of game I'm making this ruling - the general trumps the specific, here. On an effective total HT+bonuses of 15 or less, 17-18 is a critical failure and death. On an effective total HT+bonuses of 16 or higher, 17 is a normal failure (and thus mortally wounded) and 18 is a critical failure.
I don't think that's different from the rules intend, and I don't feel it's unfair or even unduly harsh. And that's how it'll go.
And does that all mean guys who bought HT 13 + Hard to Kill 2 and then get Fit and increase HT to 14 have effectively no effect from the second level of Hard to Kill? Yes. This is what I meant about over-patching.