The incident took place in Washington, US. The user has reported that his 10 month old iPhone X has exploded while updating to ios 12.1. The user of the phone has tweeted about the incident with the pictures of exploded phone.
Apple support has responded to the tweet by telling this is not an expected behaviour and apple will further look into the issue to make sure it won't repeats.
The gadgets.ndtv.com has contacted the user and the following was his response
Mohamad has also told that apple has asked him to ship the phone back so that they can investigate further on the issue.
This looks like a rare occurrence but still it is a serious issue that has to be resolved.