The Nine Mystery by Antonio Fumarola

The magician shows/writes the number "nine" (a letter and not a number) on his smartphone, paper or business card, shows it and, pointing out that there is a grammatical error, places it upside down on the table (or in the hands of the viewer). The grammatical error is the lack of the dot on the "i" of "nine" and to be able to solve it, draw the dot on another sheet which will magically disappear and then reappear on the writing shown previously ("nine"). Bug fixed like magic!