MerakSpielman said:
Allow me to present a scenario:
We have a 5th level Fighter with 14 CON and 50 hp.
So, in other words, he's rolled on average 9.5 for each level-up. There's a small chance that he's rolled a 1 somewhere along the line, since he must come up with 38 hp on 4 rolls, or 35 hp on 3 rolls (assuming a single roll of a 1). Note that in no case could he have rolled multiple 1s, since then he'd need 32 hp on 2 rolls.
Summary:
CL | New HP | Total HP | 50 less HP
1 | 12 | 12 | 38
2 | 3 | 15 | 35
3 | 3 | 18 | 32
4 | 12 | 30 | 20
5 | 12 | 42 |
8
So, we'll assume he has max 1st-level HP, and average after that:
CL | New HP | Total HP | 50 Less HP
1 | 12 | 12 | 38
2 | 10 | 22 | 28
3 | 9 | 31 | 19
4 | 10 | 41 | 9
5 | 9 | 50 | 0
Let's say now that he loses 4 points of CON, due to poison or whatever. He loses 2 hp per level, and now has 40 hp, right? I think we can all agree on that.
Correct. In fact, what happens is he changes to:
CL | New HP | Total HP
1 | 10 | 10
2 | 8 | 18
3 | 7 | 25
4 | 8 | 33
5 | 7 | 40
Note that, in this case, he loses 10 current and 10 max HP, because the reduction in Con bonus does not cause any individual HD roll to hit the 1 point minimum.
But what happens if he is already injured? Say our dear fighter has been reduced to 10 hp, and THEN he loses 4 points of CON.
He loses, again, 10 current and 10 max HP, dropping him to 0 / 40. He's staggered.
If he was already staggered (at 0 / 50) when the Con damage was dealt, he'd drop dead immediately.
EDIT:
OK, for fun, we'll assume that the Fighter 5 with 50 HPs had been unlucky once while rolling his HP, but very lucky the rest of the time. His hit point progression now looks like:
CL | New HP | Total HP | 50 less HP
1 | 12 | 12 | 38
2 | 3 | 15 | 35
3 | 12 | 27 | 23
4 | 12 | 39 | 11
5 | 11 | 50 | 0
Now, instead of being hit with a 4-point Con damage attack (which is just enough to set his Con bonus to 0), he's instead hit with a 6-point Con damage attack. What happens? The knee-jerk reaction is that he's now at 50 - (3 * 5) = 35 Max and Current HPs (and, generally, this formulation will work).
However, that 3 at 2nd-level causes a bit of a problem, since it cannot be reduced to less than 1. So, what really happens is his HP progression changes to:
CL | New HP | Total HP
1 | 9 | 9
2 | 1 | 10
3 | 9 | 19
4 | 9 | 28
5 | 8 | 36
Similarly, another 2 points of Con damage will only reduce his max and current HP by 4, since his 2nd-level HP have bottomed out.