First, since the AST is so simple to walk programmatically, one could write a simple check for that kind of bug in, I'm guessing, a few lines of Lisp. Second,

First, since the AST is so simple to walk programmatically, one could write a simple check for that kind of bug in, I'm guessing, a few lines of Lisp. Second, just because you can come up with a bug that can be written in a language doesn't mean the language doesn't prevent all sorts of other bugs. All languages can have bugs written in them. That's not the point.