I wouldn't say that the number of errors in code directly depends on complexity of the language. From my practice I learned that the code quality depends on a software development process implemented in your team. Now we have whole bunch of practices and tools that allow us to minimize risks of appearing stupid mistakes in code: covering the code with unit and integration tests, static and dynamic code analysis, continuous integration, code reviews, refactoring and so on, and, I should say, these things work. So, I think, this article demonstrates that still there are a lot of teams whose development process can be improved.