Bismillah,

It turns out the infallible is not infallible, read:

In al-Kafi vol.7 pg.203 we see al-Hassan (ra) wipeout his own infallibility from existence:

عِدَّةٌ مِنْ أَصْحَابِنَا عَنْ أَحْمَدَ بْنِ مُحَمَّدِ بْنِ خَالِدٍ عَنْ عَمْرِو بْنِ عُثْمَانَ وَ عَنْ أَبِيهِ
جَمِيعاً عَنْ هَارُونَ بْنِ الْجَهْمِ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ قَالَ سَمِعْتُ أَبَا جَعْفَرٍ وَ أَبَا عَبْدِ اللَّهِ ( عليه السلام ) يَقُولَانِ بَيْنَا الْحَسَنُ بْنُ عَلِيٍّ ( عليه السلام ) فِي مَجْلِسِ أَمِيرِ الْمُؤْمِنِينَ ( عليه السلام ) إِذْ أَقْبَلَ قَوْمٌ فَقَالُوا يَا أَبَا مُحَمَّدٍ أَرَدْنَا أَمِيرَ الْمُؤْمِنِينَ ( عليه السلام ) قَالَ وَ مَا حَاجَتُكُمْ قَالُوا أَرَدْنَا أَنْ نَسْأَلَهُ عَنْ مَسْأَلَةٍ قَالَ وَ مَا هِيَ تُخْبِرُونَا بِهَا فَقَالُوا امْرَأَةٌ جَامَعَهَا زَوْجُهَا فَلَمَّا قَامَ عَنْهَا قَامَتْ بِحُمُوَّتِهَا فَوَقَعَتْ عَلَى جَارِيَةٍ بِكْرٍ فَسَاحَقَتْهَا فَأَلْقَتِ النُّطْفَةَ فِيهَا فَحَمَلَتْ فَمَا تَقُولُ فِي هَذَا فَقَالَ الْحَسَنُ ( عليه السلام ) مُعْضِلَةٌ وَ أَبُو الْحَسَنِ لَهَا وَ أَقُولُ فَإِنْ أَصَبْتُ فَمِنَ اللَّهِ ثُمَّ مِنْ أَمِيرِ الْمُؤْمِنِينَ ( عليه السلام ) وَ إِنْ أَخْطَأْتُ فَمِنْ نَفْسِي فَأَرْجُو أَنْ لَا أُخْطِئَ إِنْ شَاءَ اللَّهُ

Some of our companions from Ahmad bin Muhammad bin Khaled from ‘Amr bin ‘Uthman and from his father all of them from Haroon bin al-Jahm from Muhammad bin Muslim that he said: I heard abu Ja’afar (as) and abu ‘Abdullah (as) say: When al-Hassan bin ‘Ali (as) was present in the Majlis of Ameer al-Mumineen (as) some folks came near him, he asked them: “What is your need?” they said: “We were searching for Ameer al-Mumineen (as)” He said: “What do you want from him?” they said: “We wanted to ask about an issue.” he said: “Well what is it? tell me.” They said: “A woman had intercourse with her husband and then she went and did it with a virgin female servant of hers, so the sperm of that man was transmitted to that young girl and she was pregnant, what do you say about this?” He (as) said: “This is a big problem that abu al-Hassan can handle, I will say that if I am correct then this is from Allah and from Ameer al-Mumineen (as) and if I am wrong then it is from my self so I hope that I will not make a mistake god willing.”

Grading: Majlisi “Sahih” Miraat 23/309.