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We're playing Hoard of the Dragon Queen, and during the brutal first episode, there is a duel that can take place (spoilers in the link).

When the player loses the duel, the other participant...

strikes one more time [and] inflicts one death roll failure on a character.

When this happened - the player marked off two death failures on their sheet. I asked them why. Here are the relevant parts of the PHB.

The section titled "Dropping to 0 Hit Points" states:

If damage reduces you to 0 hit points and fails to kill you, you fall
unconscious. This unconsciousness ends if you regain any hit points.

And later:

If you take any damage while you have 0 hit points, you suffer a death
saving throw failure. If the damage is from a critical hit, you suffer
two failures instead. If the damage equals or exceeds your hit point
maximum, you suffer instant death.

Lastly, the Unconscious condition description says:

Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.

In conclusion, any melee attack made within 5 feet of me is an automatic crit. So does that mean if a bandit is being rude, and stabs me again with his puny dagger - I have a 50% chance of dying come my turn (save for help)?

Am I right in assuming that this is hand-waved in this adventure as it specifically states "one death roll failure"?

This is an exception to the general rule, the PC takes one death save failure.

You've accurately quoted the relevant general rules.

RAW two death save failures would normally be taken by a PC from an attacker within 5 feet while that PC is unconscious (as correctly reasoned in the question).

So, if no one successfully intervenes before the PC's next turn (initiative order / luck may not be your friend), there is a reasonable chance for that PC to die. However, it's a 45% chance of death, rather than 50% (1-9 = failure, 10-20 = success).

In this instance:

The attacker is capable of dealing both melee and ranged damage. So a successful strike to an unconscious PC would normally deal either 1 or 2 death saving throw failures, depending on the attackers distance from their unconscious victim. However here it explicitly states that he 'he inflicts one death roll failure'.

This is a direct contradiction of the normal rule. In D&D 5e contradictions are reconciled via the rationale of 'specific beats general'. Clearly this is an extremely specific instance - it's one NPC, in one situation in one particular module who does something that appears contradictory to the general rule.

So, follow the specific direction given here, and not the general rule.

But why break the general rule?

The question answer is above, the following is speculation:

My hunch is that this exception was made so as not to force a certain playstyle on tables.

Whether character death is something to be encouraged or actively avoided is very table dependant. Many tables may even operate on the unspoken understanding that unconcious PCs are not attacked.

Forcing two death saving throw failures on a PC at one of these tables could be hugely controversial (due to the relatively high chance of subsequent death that you've pointed out).

Dictating that only one death saving throw failure occurs is a kind of workaround for this. It still let's the DM build up the picture of a particularly deadly / nasty NPC who will hit someone that's already down, but vastly reduces the likelihood of a PC actually dying (now at 5% on the first roll - the chance of rolling a natural 1 and taking two failures).

As suggested by Miniman, this exception could also just be an error - HotDQ was written and edited while the rules were still being developed.

Either way, if it's appropriate at your table, as a DM, you can choose to overrule this specific direction and hand out two death saves, as per the general rules.