[TillE]

Yeah. Deaths from undiagnosed heart conditions make the news every so often when it happens to young athletes. Sometimes very young. I believe just in the past few years, defibrillators have become standard equipment at football grounds in Spain and England.

[sliverstorm]

Undiagnosed conditions aside, there is also commotio cordis, which I believe has killed a number of high school sportsmen.

[rytis]

Commotio cordis (Latin, "agitation of the heart") is an often lethal disruption of heart rhythm that occurs as a result of a blow to the area directly over the heart (the precordial region), at a critical time during the cycle of a heart beat causing cardiac arrest. It is a form of ventricular fibrillation (V-Fib), not mechanical damage to the heart muscle or surrounding organs, and not the result of heart disease. The fatality rate is about 65%. https://en.wikipedia.org/wiki/Commotio_cordis