One Night Stand
Nina has a night out with Lily and the two get intimate in her bedroom at the end of the night. Ironically, this never happened; Nina only hallucinated it.
Black Swan
Thomas tells Nina that she does not have what it takes to perform the role of the Black Swan. Ironically, he casts her in the role anyway.
Doppelganger
Nina has stabbed the doppelganger attacking her in her dressing room in order to perform the Black Swan part. Ironically, Nina has stabbed herself as she has hallucinated this.
Age
Beth has been forced out of being the prima ballerina by Thomas. Not because she isn't good enough, but ironically it is because she is too old, along with being temperamental.
Lead
Nina comes to Thomas in his office to ask him for the lead role in Swan Lake. Ironically, instead of dealing professionally with her he comes on to her and kisses her in order to see how far she will go.