Common mistakes I've seen in 6 years of teaching RC.
(Trying) to use thick CA to install fabric hinges. Sorry it has to be thin in order to wick into the gap.

Not enough rubber bands on the wing.

Forgot that little black screw that holds the black servo arm safely on the servo. The record was 3 out of 4 on a Kadet came off on ONE flight. Thank goodness the LT 40 is a great rudder only airplane!

Forgetting to charge one or both batteries before you head out

Not charging things long enough

Installing the _(%&#)(&#$ paper gasket between the engine and the muffler. LEAVE IT OFF! The first thing that happens is the gasket compresses, the nuts on the muffler loosen up and at the end of the first flight you've lost a 45 Dollar muffler.

And under the issue of over controlling above, just remember that virtually all model trainers out here today, if properly trimmed, will fly just fine all by themselves, all we do is mess them up. So keep your messing up inputs to a minimum and your airplane will fly just fine.

When it comes time to land, sometime in that last foot of being airborne, STOP looking at the airplane! WATCH the WHEELS! fly them down to the grass or runway.

Oh the absolute WORST mistake ever done by a nubie? I had an impatient 12 year old that though he could build a KADET by just looking at the pictures. When I did the detailed preflight I noticed that I could twist the tail feathers to the left and right by a considerable amount. Then I noticed the shifting under the covering of the fuselage between the wing and the tail. In an LT 40 there is a picture showing you to use rubber bands to hold the fuselage together. The words then say to glue everything in place. He left the rubber bands, didn't glue any thing and figured the covering would hold it all together. He also didn't have the patience to fix it.

Good luck in your quest and have LOTS of fun

Tom