Kitāb al-Ghayba > Concealed Birth of the Imam of the Time (a.s) was not an extraordinary event
Hadith #4

وأما إنكار جعفر بن علي عم صاحب الزمان عليه السلام شهادة الإمامية بولد لأخيه الحسن بن علي ولد في حياته ودفعه بذلك وجوده بعده وأخذه تركته وحوزه ميراثه وما كان منه في حمل سلطان الوقت على حبس جواري الحسن عليه السلام واستبدالهن بالاستبراء لهن من الحمل ليتأكد نفيه لولد أخيه وإباحته دماء شيعتهم بدعواهم خلفا له بعده كان أحق بمقامه فليس بشبهة يعتمد على مثلها أحد من المحصلين لاتفاق الكل على أن جعفرا لم يكن له عصمة كعصمة الأنبياء فيمتنع عليه لذلك إنكار حق ودعوى باطل؛ بل الخطأ جائز عليه والغلط غير ممتنع منه.

As for the acts of Ja’far Ibn Ali, the uncle of the Master of the Age, his rejection of the testimony of the Imamiyah that his brother, Hasan Ibn Ali had a son born in his lifetime, his rejection of his existence after his brother, his usurpation of his brother’s inheritance, his behest to the rulers of the time to imprison the concubines of Hasan in order to force them to negate pregnancy to underscore his rejection that his brother had a son, and his declaration that any Shia, who claimed that Hasan left behind a successor was worthy to be killed, these cannot confuse a learned man, because everyone agrees that Ja’far was not infallible like prophets, that had he been so, it would have been impossible for him to reject the truth and uphold the wrong. Rather, he was fallible and able to make mistakes.