If you instead hear the Marimba ringtone, you have the bug. Marimba is the iPhone's "built-in" default ringtone, indicated by the word "Default" in parentheses after its name. If you have the default ringtone bug, any changes you make to the default selection do not stick. Even though a checkmark may appear next to some other tone, Marimba is still the one that plays.
One exception: If you set a custom ringtone for a particular person in your Contacts list, as done by editing the entry in Contacts, the chosen tone should play as expected--whether you have the bug or not. It's only if the selection remains "Default" that you always get the Marimba tone.
Several threads in Apple's Support Communities discuss this symptom and its possible cures. The consensus solution, which one thread claims was recommended by Apple tech support, is to:
(1) Force close all apps listed in the iPhone's multitasking bar (a basic technique explained in an ).
(2) Power down and restart the iPhone.