According to a study (2013) fish bone ingestion by young kids is rather common.
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?