When Stefan shows Elena the corsage he got for her, it is a plain white rose. When he pins it to her dress, the corsage has fabric and bead embellishments.
As Stefan is leading Elena into the deserted dance gym, he mentions that the last time they were there was when Klaus forced him to turn off his emotions in 'The Reckoning'. While they were all actually in the cafeteria when Stefan turned his emotions off, he specifically says that they were in the gym that night, which is true, not that they were in the gym when it happened.