Fidelio
Beethoven made several changes to the opera at different times.  This included writing four different overtures.  They are called "Leonore 1", "Leonore 2", "Leonore 3" and "Fidelio".  The overture "Fidelio" is the one that is normally heard now at the beginning of the opera.  "Leonore 2" was the overture performed at the first performance in 1805.  "Leonore 3", composed in 1806, is the most famous of the overtures.  "Leonore 1" may have been written in 1805, or it may have been intended for a performance in 1807 in Prague which never took place".  Sometimes the "Leonore 3" overture is played during Act II between the two scenes.  However, many people think it does not work dramatically there.  It is usually heard today as a separate concert piece.  The overture "Fidelio" was composed for performances of the opera in 1814.