I was thinking about the Great Weapon Master feat (PHB, p. 165), part of which reads:

On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

And the Undead Fortitude trait of Zombie creatures (Monster Manual, p. 316):

Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

If a PC with the GWM feat drops the Zombie to 0 hit points with an non-crit, non-radiant damage attack and the Zombie succeeds on the saving throw, does the PC get another attack as a bonus action?

You don't get a second hit

Undead Fortitude says (emphasis mine):

On a success, the zombie drops to 1 hit point instead.

This indicates that Undead Fortitude interrupts damage. Similar to damage reduction, Undead Fortitude activates after the hit but before damage is taken. Thus, the undead never reaches 0 HP if it passes the Constitution Save, and the GWM bonus attack isn't activated.

Order of events:

1. You make a lethal hit


2. The undead makes a save


3. It's HP becomes 1


4. GWM isn't activated.

If the ability said something similar to "the undead recovers 1 HP on a success" then he would have dropped to 0 HP and triggered GWM.

Compare to Shield and the Monk's Deflect Missiles:

Shield:

1 reaction, which you take when you are hit by an attack

...

Until the start of your next turn, you have a +5 bonus to AC, including against the triggering [a]ttack.

Deflect Missiles:

you can use your reaction to deflect or catch the missile when you are hit by a ranged weapon [a]ttack.

Both of these abilities interrupt their trigger and have the potential to prevent the incident that triggered them.

Hit 'em baby, one more time.

The most important bit of Undead Fortitude is:

If damage reduces the zombie to 0 hit points

There is clearly a gate where the Zombie is at 0HP. At that point, they can make their save and possibly continue with 1 HP, but they have cleared the 0HP gate to initiate the GWM bonus action melee attack.

The zombie may finally end up at 1HP, but that doesn't change that they were reduced to 0HP and then made a save to finally end up at 1HP.

GWM only requires that you

reduce a creature to 0 hit points with one [melee attack].

And you have done this, it just didn't stick :)

The progression went from:

1. Zombie at more than 0HP


2. GWM hit succeeds, reducing zombie to 0HP.


3. GWM allows for bonus action attack from reducing to 0hp.


4. Zombie makes CON save and succeeds. HP now at 1.


5. GWM can be used for bonus action attack to attack again because of Step 3.

Comparison with Avoiding Disintegrate by saving at 0HP to stay at 1HP.

The following errata answers what happens when a Barbarian with Relentless Endurance can avoid disintegration because they can avoid 0HP with a successful save.

Q: If the damage from disintegrate reduces a half-orc to 0 hit points, can Relentless Endurance prevent the orc from turning to ash?

A: If disintegrate reduces you to 0 hit points, you’re killed outright, as you turn to dust. If you’re a half-orc, Relentless Endurance can’t save you.

It's clear that the save won't save them, as they have still reached the 0HP gate that Disintegrate dictates.

GWM and Undead Fortitude fall under the same gate reaching of 0HP.

To add a bit more potential confusion, Wizards introduced another errata about Disintegrate changing the last line to:

The target is disintegrated if this damage leaves it with 0 hit points.

I believe that this fundamentally changes their errata above in that the newer errata clearly states that the half-orc is not left at 0 because of Relentless Endurance. Disintegrate is not leaving the half-orc at 0, because that race feature changes the end state.

This changes how Disintegrate works with the half-orc feature, but does not change the 0 HP gate from GWM. The updated language simply alters this spell/race example and not the GWM/Undead Fortitude interaction.

The narrative

A way of looking at this is the Zombie is brought down with the hit. It's makes the save and rises again from the (un)dead. But at one point, the zombie was at 0HP and down.