As a PHP programmer, I actually write code in many different languages. Many times in a single file I will have (X)HTML, CSS, Javascript, PHP, and SQL. Most of the time, I can keep this straight. Semi-often I forget to put a semi-colon at the end of a line (like when I do a bunch of Python programming the night before). But there are certain instances where I get tripped up all the time: while, for, and foreach.
Those three little words are very powerful, and are fundamental constructs of programming languages. I don’t know what I would do if I couldn’t iterate over an array. I also don’t know what to do with myself when I forget how each is used in the current language I’m writting in.
For example: If I’m writting in PHP, it’s foreach ($array as $val => $key), in Python it’s for $i in range(10):, in Perl for and foreach are actually the same thing and can be used interchangably (I did a bunch of Perl stuff while at my last job), and I’m pretty sure JavaScript doesn’t have an object iterator construct—probably because JavaScript implements objects in the most terrible way.
What about whiles? They are just strange. I never know when I can use do or when the truth statement is processed. Plus, sometimes the thought of an infinite loop scares me a little. Of course in Perl 6, you can form a lazy infinite loop out of a for statement.