I've noticed that there seems to be a class of mistakes that new programmers without CS backgrounds tend to make, that programmers with CS backgrounds tend not to. I'm not talking about not understanding source control, or how to design large programs, or a whole host of other things that both freshly minted CS graduates and non-CS graduates tend to not understand, I'm talking about basic mistakes that having a CS background will prevent a programmer from making.

One obvious and well trod example is that folks who don't have a basic understanding of formal languages will often try to parse arbitrary HTML or XML using regular expressions, and possibly summon Cthulu in the process.

Another fairly common one that I've seen is using common data structures in suboptimal ways like using a vector and a search function as if it were a hash map.

What sorts of other things along these lines would you look out for when on-boarding a batch of newly minted, non-CS programmers.