The perennial problem of using secondary sources is that you don't know which sources they used. In practice, most authors copy from each other because it's much easier than digging out the primary sources. So mistakes get handed down like heirlooms.
I have traced that '15mm MG 151 in the Bf 109's cowling' error back to William Green in the 1960s. Perhaps it was his mistake, perhaps he got it from somewhere else. It's been shown to be an error long since, but it keeps chugging along
What you need to do is to find an author who takes an active interest in the specific aspect of the subject. As I'm a gun and ammo specialist, I'm more likely to make mistakes in dealing with the aircraft than their armament. I've always found Manfred Griehl to be very good on the armament of German planes, Bill Gunston is usually reliable on British ones.
Tony Williams: Military gun and ammunition
website and discussion
forum