Continuity mistake: When Niles and Daphne are on the roof having their picnic, Niles tries Daphne's peanut brittle but when she turns away he throws it off the roof, and lays the plate down on the blanket. When Daphne turns back and asks him what happened to the candy, we see him holding the plate again, even though he had no time to pick it up.
Continuity mistake: When Daphne and Niles are on the roof having their picnic, the basket behind them is opened from the center and on the inside on the left are two champagne glasses. A couple of shots later and the glasses are now on the right side, even though no one touched the basket.
Continuity mistake: When Niles and Daphne are on the couch talking about the meteor shower, she has his hand on his left knee and he is covering it with both of his hands. After he says "two horrible wrongs", his right hand is suddenly on his right knee, with no time for natural switch.