Red Flags in a programmer

You know you might be dealing with a `coder' when he/she says:

I'd rather not reinvent the wheel

Open a CL REPL and input:

(+ 5 "aaa")

You'll get something like

debugger invoked on a SIMPLE-TYPE-ERROR in thread ...

That, my friend, is a type error. In a unityped languge Common Lisp. And no matter how "dynamic" a language is, it will still be vulnerable to type errors, because not every value supports all possible operations.