Why did my phone lose the ringtone configuration? I've assigned a default ringtone and one other ringtone to my wife. She called me two days ago and the phone played the Galaxy SII ringtone. Next day, somebody else called me, and the same happened.

So, I assigned the ringtones again, but I was wondering if this happens often, does it happen to others, is there a reason it happened to me (did I do something wrong)...

Probably it's a bug. Last night, the OS updated itself, and the ringtones were not affected.
I don't know if it could be your case, but if your phone is connected to your computer with UMS (USB Mass Storage) enabled along having a custom ringtone stored on your SD card (/ringtones folder) instead of MTP this will give you this kind of problem.

The phone will use a default ringtone since it cannot access the SD card files, because the UMS mode requires exclusive access to the mass storage.