@sonhouse saidOften ignored problem, but very real. Important to note, of course, that an individuals' DNA is not static or uniform; every time a cell divides, DNA replication makes an error every 10 million base pairs. Many of these get repaired, but you still end up with at least 1 mutation per cell division. Interestingly you can sequence the same genome and somehow the same mistake is made. Lots and lots of papers have been published that claim specific polymorphisms "cause" disease but the mutation might not even be there.
https://phys.org/news/2019-01-long-read-dna-analysis-errors-experts.html