I have seen and ignored a number of typographical or other errors on the site, particularly from non-members who have no opportunity to correct an error. Joining is free and easy, and permits editing. I am always grateful to those who have found my errors-occasionally anyone can miss or make a typo.
I do not believe others should ever be allowed change caption data on photos not their own. That could get chaotic and argumentative. I had one very early example of an email criticism that was in fact incorrect. There is the better way-just contact the member via the site and be sure of your facts, perhaps with a reference, if appropriate.