Someone can correct me if I'm wrong, but I believe this is false. You have to be pertinacious in your heresy to be automatically excommunicated. An accidental heretic is still within the Church.
If these quotes are not appropriate let me know. Firstly, I don't know what an "accidental heretic" is. Nor, do I understand what "pertinacious" heresy is.
If you have a dogmatic pronouncement explaining them it would be helpful.
Here are some quotes to illustrate that a heretic is outside the church.
Pope Eugene IV, Council of Florence, "Cantate Domino," 1441:
"Therefore the Holy Roman Church condemns, reproves, anathematizes and declares to be outside the Body of Christ, which is the Church, whoever holds opposing or contrary views."
Pope Pius XII, Mystici Corporis Christi (# 23), June 29, 1943:
"For not every sin, however grave it may be, is such as of its own nature to sever a man from the Body of the Church, as does schism or heresy or apostasy."
Pope Leo XIII, Satis Cognitum (# 9), June 29, 1896:
"The practice of the Church has always been the same, as is shown by the unanimous teaching of the Fathers, who were wont to hold as outside Catholic communion, and alien to the Church, whoever would recede in the least degree from any point of doctrine proposed by her authoritative Magisterium."
Pope Leo XIII, Satis Cognitum (# 9):
"No one who merely disbelieves in all (these heresies) can for that reason regard himself as a Catholic or call himself one. For there may be or arise some other heresies, which are not set out in this work of ours, and, if any one holds to one single heresy he is not a Catholic."
from:
http://www.mostholyfamilymonastery.com/Sedevacantism_Refuted.htmlMy understanding is that even if you don't know you are a heretic, you can still be a heretic. Its like if I didn't know I had to be baptized to be saved, I still wouldn't be saved. In orer to not be a heretic you have to know what you need to know to be saved.
Am I wrong?