Wikipedia (https://en.wikipedia.org/wiki/Code_review) says: "Empirical studies provide evidence that up to 75% of code review defects affect software evolvability/maintainability rather than functionality, suggesting that code reviews are an excellent tool for software companies with long product or system life cycles. This also means that less than 15% of the issues discussed in code reviews are related to bugs.

So, "don't usually" equals 15%. :^)

Deep dive: https://www.michaelagreiler.com/wp-content/uploads/2019/02/C...