The problems begin when the ability-distribution of red/blue weasels change, and the hiring-committee is still using restrictions based on the old distribution. eg red weasel ability has been steadily increasing, but the old hiring criteria still says "don't hire red weasels as they have no technical ability to speak of!"

but yes I agree - it's all difficult because it's hard to create a test that is as accurate as actual real-life working with a person. That's why the popularity of those awful "three month probation periods".