Emperess Elisabeth's nickname was never "Sissi". It was Sisi, with one S, and she signed herself as "Lisi".
In the movie Franz Joseph and Elisabeth get married in the St. Michael's Church in Wienna. In real life they got married in the Augustiner church.
There is no historical proof that Emperor Franz Joseph actually supposed to marry the Dutches Helene in Bavaria, Sissi's older sister.
Sissi's mother mentioned that Helene met Maximilian Anton von Thurn und Taxis. The scene is set in 1853 while in real life they met in 1856.
The Emperor Waltz is played as background music during Sisi and Franz Joseph's wedding dance in 1854. The Emperor Waltz was composed by Johann Strauss II in 1889.
Colonel Böckl is watching Sissi climbing out of a window of her bedroom where she has been locked just before by her mother and now wants to get and and go fishing. Böckl is hiding behind a bush and several scenes show him with the town in the background. In one scene for a very short moment you can clearly see a car passing by far between two houses.