When Caroline is telling Stefan about Matt, Stefan tells her to compel him and asks if the vervain is out of his system. Caroline says she usually slips it into his soda but didn't get to the night before, so it should be out of his system. All other times it takes three days for the vervain to leave someone's system.
Elena is giving a speech and then John falls down the stairs. She walks over to see what is going on but is interrupted by Katherine - but somehow they've already swapped clothes at the point of interception. Elena starts off wearing a purple dress with her necklace, but suddenly it's Katherine that is wearing the purple dress but with no necklace and Elena is wearing a black dress with a necklace.
When Katherine intercepts Elena after her speech, they haven't already changed clothes. Katherine is wearing a purple shirt with buttons down the front and Elena is wearing a purple dress with no buttons. This is also why Katherine isn't wearing the necklace yet.
When Stefan and Damon arrive at Isobel's house ("the nicest foreclosure in town"), they walk inside into the front hall and pause to decide where to go next. In a long shot we see the front door suddenly swing inwards by itself and stop just short of closing. This happens at least 30 seconds after the brothers have passed through it, the position of the door means it could not have been a gust of wind, the timing means it could not have been a normal spring hinge action, and the Salvatores are shown at least 2 feet away from it when it starts to move. In addition: in the very next shot when Damon walks past the door, it is back to being half open, and the shot after that the brothers walk out and Stefan slams the (now completely open) door shut.