In programming, as many professions, there are mistakes you'll make now that you may not have made as a beginner. For instance, consider pointers:

In C++, is there any difference between arrays and pointers?

The beginner will say yes, they are two completely different concepts, and treat them as such.
The intermediate programmer (having learned that arrays are pointers internally) will tell you no, they are the same thing, and may miss some crucial bugs by interchanging them all willy-nilly.
The expert, however, will again say they are different things (ex. see here or here).

What other questions/mistakes are like this?