May I please ask you a question ?
If one does an aveirah, my understanding is that he only needs to do teshuvah.
1) why only by a twisted retzuah do the acharonim (starting with the Magen Avraham) mention a custom to additionally fast ?
2) what exactly is the severity of this inadvertent sin over all others ?
Thank you for your question.
There are many levels of teshuva, and the higher the level, the cleaner the person’s neshoma will be. This is similar to clothing that got dirty, not all stains come out in the wash, some only get lighter, and come need stronger detergents, etc. There are numerous aveiros that it is brought down that the person should fast. Fasting is associated with teshuva in many aspects, for example, on Rosh Hashana ותשובה ותפילה וצדקה, in many machzorim the word צום is written on top of the word teshuva. The seforim do write regarding certain severe aveiros, such as chillul Shabbos etc. They even discuss how many fasts a person should fast. Nowadays we don’t do this because we are weak, and the fast may adversely affect us, as we might become irritable and get angry or we won’t be able to learn, therefore it isn’t mentioned.
Regarding your question, according to some Achronim it is simply because the person then said a bracha l’vatala. (The fact that we often do so without taking notice doesn’t take away from the severity of making a bracha l’vatala, rather it means that we have to be more careful about it.) Others say that it is only if it was left like that intentionally, but not if it was mistakenly happened. Additionally, this whole idea is not an obligation, but something for pious individuals, but not for the average person.
Eliyahu Rabba 27-13 in the name of Rabeinu Yirucham, M:B 27-38, Piskei Teshuvos 40 ftnt. 9.