There can be no error when posting on Catholic faith and practice. No posts about the Catholic faith are to be taken as being what a person believes or opines regarding the Church.
No one has their own opinions or beliefs, they now all have their own facts and posts are to be treated as such.
But what sound people (Protestants) post are not facts. What are we to do then?
Can we ask them for the source of their information? Why they might believe this about the Catholic Church? Was that information passed to them through a pamplet or a preacher? How do they know the preacher was a valid preacher and really knew the Scriptures? Or was the preacher using sola scriptura and voicing his own opinion?