According to a study (2013):
Ingestion of fish bones or coins by young children is a relatively common presentation in the PED, and it is associated with frequent medical interventions. Although the overall prognosis is good, and improving the health education of the population should be considered to reduce the frequency of these episodes.
Note: PED = Pediatric Emergency Department
Also, in some extremely rare cases the bone can penetrate the digestive tract and end up in other organs (2015).
Fish bones come in various sizes, hardness etc. Eating fish would inevitably result in swallowing them. Although I assume small fish like sardines shouldn't be much of a problem, bones from bigger fish can potentially be dangerous.
When would swallowing a fish-bone be a problem? (e.g. can cause injury in any part of the digestive system) Would the stomach dissolve partially or completely some fish-bones?
Also, are there any differences in whether a kid or an adult swallowed it?
Note: If it can be generalized for other bones as well, let me know. (or simply edit the question)