Revealing mistake: The first morning, when Robert takes Giselle to work, they enter atrium of the Time Warner Center (the well-known statue of the female Giselle admires is one of a set, the other is male), which is located at 8th Ave and Broadway, at the southwest corner of Central Park. Upstairs, when Sam tells Robert that she can't find where Giselle is from, and then when Giselle speaks to Phoebe, the very distinctive Johnson Lipstick Building (it's reddish and has three receding oval-shaped levels) at 53rd and 3rd is seen outside the window. The office scene is obviously not filmed at Time Warner, because it's impossible to be in an office at 8th and Broadway, on the West Side, and be next door to a building located at 53rd and 3rd, on the East Side. (00:35:40 - 00:40:35)
Revealing mistake: In the hotel room, Nathaniel picks up the squirrel by the tail. In the very first shot he goes to grab it and grabs thin air below the prince's knee. In the quick angle change he has the squirrel in his hand. It looks like either the actor or the computer animator didn't quite sync things up.
Answer: If Elsa died from magic, her son, the prince, would know and turn against her. If she died by somebody else or a tragic accident, the Queen's hands would be clean.