The question's been asked before elsewhere, but for lack of an authoritative answer:
If a character has a sufficiently small constitution score, it's conceivable that, when they use a hit die to heal during a short rest, that the roll might not be enough to offset the penalty.
The PHB doesn't have a lot to say on the matter (page 186):
A character can spend one or more Hit Dice at the end of a short rest, up to the character's maximum number of Hit Dice, which is equal to the character's level. For each Hit Die spent in this way, the player rolls the die and adds the character's Constitution modifier to it. The character regains hit points equal to the total.
For example, given a character with constitution 4 (a modifier of -3) who rolls a 1 on their hit die, so the total is -2.
I could think of a few ways to handle this situation, and arguments for each:
- have the character lose two hit points, that's what the numbers say
- have the character neither gain nor lose a hit point, the rules only mention "regaining hit points" with a hit die, not losing.
- have the character gain a hit point, they did spend an entire hit die