If someone does something against his oath due to forgetfulness
حَدَّثَنَا عُثْمَانُ بْنُ الْهَيْثَمِ، أَوْ مُحَمَّدٌ عَنْهُ عَنِ ابْنِ جُرَيْجٍ، قَالَ سَمِعْتُ ابْنَ شِهَابٍ، يَقُولُ حَدَّثَنِي عِيسَى بْنُ طَلْحَةَ، أَنَّ عَبْدَ اللَّهِ بْنَ عَمْرِو بْنِ الْعَاصِ، حَدَّثَهُ أَنَّ النَّبِيَّ صلى الله عليه وسلم بَيْنَمَا هُوَ يَخْطُبُ يَوْمَ النَّحْرِ إِذْ قَامَ إِلَيْهِ رَجُلٌ فَقَالَ كُنْتُ أَحْسِبُ يَا رَسُولَ اللَّهِ كَذَا وَكَذَا قَبْلَ كَذَا وَكَذَا. ثُمَّ قَامَ آخَرُ فَقَالَ يَا رَسُولَ اللَّهِ كُنْتُ أَحْسِبُ كَذَا وَكَذَا لِهَؤُلاَءِ الثَّلاَثِ. فَقَالَ النَّبِيُّ صلى الله عليه وسلم " افْعَلْ وَلاَ حَرَجَ " لَهُنَّ كُلِّهِنَّ يَوْمَئِذٍ، فَمَا سُئِلَ يَوْمَئِذٍ عَنْ شَىْءٍ إِلاَّ قَالَ " افْعَلْ وَلاَ حَرَجَ ".
Narrated `Abdullah bin `Amr bin Al-As: While the Prophet (ﷺ) was delivering a sermon on the Day of Nahr (i.e., 10th Dhul-Hijja-Day of slaughtering the sacrifice), a man got up saying, "I thought, O Allah's Messenger, such-and-such a thing was to be done before such-and-such a thing." Another man got up, saying, "O Allah's Messenger! As regards these three (acts of Hajj), thought so-and-so." The Prophet (ﷺ) said, "Do, and there is no harm," concerning all those matters on that day. And so, on that day, whatever question he was asked, he said, "Do it, do it, and there is no harm therein."