When Lena is in bed in Greece, the lighting on her face changes inconsistently between shots. From some camera angles, the light is coming from her left; from others, it is coming from her right; in at least one, there is light on both sides.
When Lena is talking to Kostos at the restaurant, we see a man in a sweat shirt passing by in her back. she looks in direction of her friends. After the cut, this same man is in the upper background. then after another cut he is past them.
The glasses Tibby and Brian are holding changed color - at first, Brian is holding light blue one and TIbby dark blue one, then Brian put the dark one on the floor and the light is on the table (supposedly put there by Tibby).
When Tibby is talking to Brian on his porch she has some of her bangs in front of her ear, then when the shot changes she has her bangs completely pulled back. Brian did pull back part of her bangs but ONLY part so some people may not notice but actually, its not just her bangs behind her ear, they're pulled up into her pony tail.
When Lena is at the funeral in the beginning of the movie, for someone of the Greek Orthodox faith, she makes the correct sign of the cross. The Greek Orthodox Church crosses itself from right to left rather then Roman Catholic left to right.
When Bridget finds the letters from Greta in her dad's basement that her father had hidden from her, there is no USPS cancellation stamp printed over any of the postage stamps. This would indicate the letters were intercepted before they were mailed. Since Greta lives in Alabama, this doesn't seem possible.