Adam - joking aside I would look for another instructor - I dont really care how many hours he has (or hasnt) there are some fundamentals where it is troubling to see a pilot make certain mistakes.
OK, so perhaps you forget to turn on the strobes before departure - you missed the item on the checklist and it happens, there are potentially no catch points during the flight that would alert you to the "problem".
However if you leave the flaps down there should be a number of indications that the aircraft is no performing as it should. Firstly the speed will be way adrift from the power setting, secondly the handling characteristics and probably the pitch angle will be different and thirdly if you ever carry out a visual check of the wings you cant help but notice. Now that is a whole load of things to ignore, and more to the point a whole load of things to be ignorable. The aircraft will tell you that all is not well if nothing else. I dont mean to be harsh, but if you ignore all of these signs it suggests that you are not yet familiar with the aircraft, or its performance or how it should behave; its a bigger concern because performance issues of this sort often occur for other reasons and therefore are indicative of more serious problems.
Now I am not suggesting that we havent all "forgotten" to retrack the flaps at the appropriate point after departure - that is one thing, but once settled in the crusie for any time, if you havent identified the issues I have suggested and at the very least connected the lack of performance with the flaps still be deployed you should be concerned.
Probably like the rest of us I have done it but didnt find myself wondering why the aircraft had accelerated in the cruise!