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?