[cantlin]

In some circumstances "accumulation of knowledge" in a tester is exactly what you're trying to avoid. Fresh eyes can be very valuable. Amongst other reasons, this is why we don't give developers sole responsibility for testing their own work.

[andybak]

That's an interesting point but I think it highlights that there are different types of testing - even above the obvious UX vs QA distinction.

I find that really good QA testing often needs someone quite technical and even someone who has some insight into the way the code works. You have to actively think about ways the system could break. It's probably closer to the mentality of a penetration test than it is to usability testing.