For example, in this day and age of fast processors, abundant memory, large hard-drives, you can't expect a novice to put much emphasis on efficiency. He'll code and will run his program just to see the output appear almost instantly. A veteran will look at it and shake her head, but that's because she knows that back in her days, code this inefficient guaranteed that you'd spend the day in the lab just waiting to see the first letter of the output. Don't expect the newbie to learn that lesson any time soon, with his blazingly fast laptop, on which he's working comfortably from his bedroom. Now, hand him one of those snail computers at his next internship and watch the transformation operate.
The difference is that an expert will be able to make a good decision about that trade-off. A novice is restricted to the tools and practices they are familiar with - they might come to the same conclusion, but not for the same reasons.
But the manifestation more relevant to the real world is not having the experience base to realize it's biting you in the ass. Specifically, even though X is working, it could be working better, for less effort. Or another common case is X is not working and you misidentify why ("lazy developers not willing to do the work" instead of "used the wrong paradigm for the job", for instance).