Critics, scholars, and performers have long noted that Arturo Toscanini’s reputation for absolute fidelity to the printed score was little more than a public relations myth.
Now that the legendary conductor’s annotated scores are available for study, three types of alterations can be observed: (1) modifications of dynamics, articulation, bowing, phrasing, and tempo; (2) orchestrational adjustments; and (3) the introduction of new material.
The combination of Toscanini’s Italian musical heritage and Wagnerian aesthetic convinced him that the highest service that a conductor could render was to impose certain types of musical changes whenever he sensed that a composer’s artistic conception was threatened. In his mind, there was neither egotism nor hypocrisy in this approach.
This according to “Toscanini and the myth of textual fidelity” by Linda B. Fairtile (Journal of the Conductors Guild XXVI/1–2  49–60).
Today is Toscanini’s 150th birthday! Below, his recording of the first movement of Beethoven’s ninth symphony, one of the works discussed in the article.
Although Arrigo Boito devoted 56 years to the composition of his Nerone, at his death the opera was still incomplete; Arturo Toscanini bustled to refine and finish the last act for the work’s premiere at La Scala on 1 May 1924.
Since the figure of the mad psychopath Nero is best remembered in the collective imagination as he plays and sings while observing the Great Fire of Rome, for the first staging of the opera a true kithara was made by the lute maker Piero Parravicini at the Milan workshop of Antonio Monzino e Figli; today the instrument is on display at the Civico Museo degli Strumenti Musicali in Milan.
This according to “‘Or che i Numi son vinti, a me la cetra, a me l’altar!’: Kithara constructed for the premiere of Arrigo Boito’s Nerone” by Donatella Melini (Music in art XL/1–2  pp. 267–72).
Above, the instrument in question (click to enlarge); below, the scene referred to in the article’s title.