"I think about 1 in 10 people are thought to have a parent who actually isn't their biological parent."
It may not be as high as that, but it certainly exists.
Going really off-topic here, but there was a study done in 2022 that surveyed 23,000 people who used the DNA services of one of the big websites and they found that around 3% of the participants discovered an unexpected biological parent.
Now that doesn't necessarily mean that the father was unaware, just that the adult child who took the DNA test was unaware. Also, in a very small number of cases it turned out that they had been adopted or were donor-conceived and never told of this.
This may be the sort of "real" upper limit for this as those fathers suspicious about their paternity may be more inclined to check it out which gives rise to higher figures in other studies
Link to the 2022 study in American Journal of Human Genetics:
"Family secrets: Experiences and outcomes of participating in direct-to-consumer genetic relative-finder services"
https://www.sciencedirect.com/science/article/pii/S0002929722000131
.
"...it's precisely why doing a test 'for fun' shouldn't be taken as lightly as it is."
This I totally agree with