Career mode teaches me not to plan my flight, and instead fly through boxes in the sky. I can’t turn off the boxes, which is good, because I get punished when I stray too far away from them.
Career mode teaches me not to do pre-flight checklists. I get punished for checking the flaps.
Career mode teaches me that it’s okay to start the engine with airport workers standing next to the propeller, and to taxi “through” people or vehicles, because it frequently doesn’t give me any other option.
Career mode teaches me that it’s more important not to exceed 20 kts during taxi (while not giving me any indication what my current speed might be until airspeed comes alive and I get dinged immediately) than to stabilize my approach before landing. It will congratulate me for the most haphazard landings, as long as I pass through the “final” checkpoint and somehow manage to stay on the runway. (Of course it will also sometimes patronize me after a perfect flight with a smooth landing, because it arbitrarily decided that, during take-off, I “overflew” the departure airport (!) without the expected radio call.)
Career mode teaches me to fly dangerously close to terrain (mostly during sightseeing missions), or enter clouds under VFR.
Career mode teaches me to treat any plane as if it was a rental and gas was free. It simply doesn’t care if I aim for best economy instead of best speed. It actually encourages me to punish the plane on every trip by telling me frequently that missions are time-critical. It’s unclear why the C172 is often considered the appropriate choice for these urgent missions.
Career mode teaches me to make useless and unrealistic radio calls (“taxi to parking” at a grass strip, or “overflying airport”) and to ignore ATC best I can. If I interact with ATC beyond what career mode wants me to do, I risk breaking the mission, and not being able to complete it.
Remember “Aviate, navigate, communicate”? Career mode teaches me to prioritize communication over anything else. If I delay my response to ATC because I’m busy with something else, I risk getting punished.
Career mode teaches me to ignore the placards telling me about gear or flap speeds. I get punished if I use the flaps at the correct speeds.
Career mode teaches me to do a 180 on the runway after landing, even if there’s another exit in front of me. I don’t think I would get punished if I use the correct exit, but at this point I’m so used to follow the blue arrows that I’d rather turn around than risk it.
I’m sure any career mode player can think of more examples.
Please see this insightful response for solutions or workarounds to some of the issues I mentioned: