The scene where Merida's dress splits, and reveals her corset, while amusing, is inaccurate given the movie's apparent medieval setting (around the 10th century). Corsets didn't come into fashion until Elizabethan times, approximately 600 years after the movie's apparent setting.
When Merida hops on her horse and takes her "day off" she is wearing a royal blue dress. However, when she climbs the rock next to the falls, and on the top of the rock, her dress is dark green. When she goes back home, she is wearing a blue dress again.
In the shooting competition, when Merida shoots her own arrow, the slow motion sequence shows that the arrow traces a small damage to her cheek. In the next scene in the castle the damage is gone again.
Merida's bow appears to be a double recurved self-wood type such as was used in the Great Plains area of North America during the 19th century. She also carries a quiver. Neither of these were used in any part of medieval Britain; bows were always self-wood (made of a single piece of timber) made without re-curves, while arrows for immediate use were carried tucked into the belt. At one point Merida carries an arrow in her mouth in order to follow up very quickly with a second shot - this was typical of native Americans of the Plains when hunting on horseback and in warfare. There are historic photographs by Edward Curtis of Plains warriors using this technique. It is definitely not recorded in medieval Scotland or elsewhere in Britain.
The goof items below may give away important plot points.
After Mor'du gets crushed by a large piece of stone, and the sun is rising, Merida removes the tapestry from Angus to place over her mother, who is still a bear, in hope to break the spell. Angus also appears to be in the circle of stones, yet, in the shot that views everyone in the circle of stones from above, Angus is nowhere in sight.
There was a severe fight between Mor'du and the Queen Eleanor who was a bear then, however, there were no bruises and any marks of cuts when the Queen turned back into a human which is quite incredible as a bear can really hurt a person very badly.