Usually the "thinks he knows everything" engineer is a poor engineer as well as a PITA. A -good- engineer wants to know where every number comes from and why it's handled the way it's being handled, but never claims to know everything or even to know better than another professional. But there are way more of the former than the latter, and one of the reasons I'm glad to be -out- of engineering.
One of the best engineers I ever worked with was very clear: "As long as proper care was taken in collecting data, then the data ARE. And if they do not agree with your theory, then it's time to re-examine your theory for the flaws that surely exist." But there are far too many numb-noggins out there who blithely and cheerfully convince themselves that their beloved theories must be so, therefore the data must be wrong. Phlogiston chemistry, anyone?