You can get this in other languages too. For example, Flask (a Python web framework) has a fantastic debug-mode error page that totally changed the way that I t

You can get this in other languages too. For example, Flask (a Python web framework) has a fantastic debug-mode error page that totally changed the way that I think about web development. Any time an exception is thrown in a view function (and this includes the exceptions that you idiomatically throw for HTTP 4xx and 5xx errors) the debug-mode error page would have a stack trace (obviously), but also an interactive REPL that could be opened at any stack frame in that trace. It wasn't necessary all that often, but when it was, boy was it a fantastic way to work.