But the balls had to strike at the exact moment indicated in the score and the players never succeeded in accomplishing that.

The passage had to be suppressed but it is preserved in the library at the Opera.

They also had to suppress the part of Catherine de Medici who should preside at the conference where the massacre of St.Bartholomew was planned.

Her part was merged with that of St.Pris.They also suppressed the first scene in the last act, where Raoul, disheveled and covered with blood, interrupted the ball and upset the merriment by announcing the massacre to the astonished dancers.
But it is a question whether we should believe the legend that the great duet, the climax of the whole work, was improvised during the rehearsals at the request of Norritt and Madame Falcon.

It is hard to believe that.
The work, as is well known, was taken from Merimee's _Chronique du regne de Charles IX_.