I do data science, but everyone who writes R has been bitten by it's scoping rules, which are definitely not mainstream. Everyone I know who writes java or python has been bitten at least once by the pass object references by value (is there a better name for this?) function argument semantics. Everyone who writes java has accidentally written == when they meant .equals and stared at the code for far too long. etc etc etc You just can't claim you can competently use a language without understanding some basics, like how functions work, how arguments are passed, and how equality is decided.