I'm not sure who GJS is but if you see any lisper editing text instead of operating on structures (with auto-balancing parenthesis and so on), it's 99% certain it's in an environment they are not familiar with, so they will make mistakes.
I don't think anyone who write Lisp-like languages professionally doesn't use tools like parinfer/paraedit, where balancing parenthesis is not something you have to do.
Yup, makes sense that you make mistakes then, as these folks are surely used to not having to think about balancing parenthesis anymore. Just as over-reliance on GPS will make your navigation skills without a GPS worse over time.
While specifically parinfer and/or paraedit might not be available in neither LispWorks or ACL editors, they surely have something there to aid with parenthesis.
But I might be wrong here, it's simply hard for me to imagine someone writing with lisps professionally going the route of textual editing when structural editing is right there.
I don't think anyone who write Lisp-like languages professionally doesn't use tools like parinfer/paraedit, where balancing parenthesis is not something you have to do.