Taanit 12B

נים ולא נים, תיר ולא תיר. דקרו ליה ועני, ולא ידע אהדורי סברא, וכי מדכרי ליה מדכר.

One is asleep but not fully asleep, awake but not fully awake. This means that if they call him he will answer, but he cannot give a coherent reason. And when we remind him of something that just happened he remembers it.

אמר רב כהנא אמר רב: יחיד שקיבל עליו תענית ⁠— אסור בנעילת הסנדל. חיישינן שמא תענית צבור קיבל עליו. היכי ליעבד? אמר רבה בר רב שילא, לימא הכי: למחר אהא לפניך בתענית יחיד.

Rav Kahana said that Rav said: An individual who took a fast upon himself is prohibited to engage in the wearing of shoes on the day of his fast. The reason is that we are concerned that perhaps he took a communal fast upon himself, and wearing shoes is prohibited on communal fast days. The Talmud asks: How should one act, to avoid this problem? Rabba bar Rav Sheila said: Let him recite this formula: Tomorrow I should be before You in the observance of an individual fast.

אמרו ליה רבנן לרב ששת: הא קא חזינן רבנן דמסיימי מסנייהו ואתו לבי תעניתא! איקפד ואמר להו: דלמא מיכל נמי אכול.

The Talmud relates: The rabbis said to Rav Sheshet, who was blind: We see rabbis who wear their shoes and go to the study house on a communal fast day. This shows that there is no need to be concerned about this prohibition. Rav Sheshet became angry and said to them: Perhaps they even ate, if you saw them treating the fast lightly.

אביי ורבא (מעיילי כי מ)סיימי אפנתא. מרימר ומר זוטרא מחלפי דימינא לשמאלא ודשמאלא לימינא. רבנן דבי רב אשי נפקי כי אורחייהו, סברי כי הא דאמר שמואל: אין תענית צבור בבבל אלא תשעה באב בלבד.

The Talmud further relates: Abaye and Rava would enter the synagogue while wearing sandals on the leather [apanta] of their shoes, i.e., they would wear their shoes upside down. Mareimar and Mar Zutra would switch the right shoe for the left and the left for the right. By contrast, the rabbis of the school of Rav Ashi would go out wearing shoes in their usual manner. They hold in accordance with that which Shmuel said: The only completely stringent communal fast in Babylonia is the 9th of Av alone. Therefore, in Babylonia there is no need to be concerned that one might have taken a communal fast upon himself.