Yeah, it happens.

I was fire-wardening at a carol service about 15 years ago (think ~850 year old gothic stone church with thousands of candles spread around on every available flat surface - looked amazing, potentially catastrophic).

I was sat near the back with a bucket of sand under my seat, and a guy sat in the middle of the row just across the aisle from me, who would have been around 40 or so, had a heart attack and died.

No warning, no history, just gone, and leaving a fairly young family behind. It was honestly shocking.