Another reason I believe it is important to document specific behaviors related to students not previously identified with a special need is that an important life event may have happened in their life. Maybe there was a death in the family, a new sibling, or some other different situation in the home. This may bring out different behaviors in the child and this would be the reason for a specific type of behavior ot occur as opposed to the normal behavior.
Lastly, another scenario might be a hereditary special needs trait. Keeping a log of these specific behaviors may help the teacher to understand the student better in order accommodate his or her needs within the classroom. The school I currently am with is NAEYC accredited and we are responsible for observing the children and their developmental progress through the goals of the curriculum. A teacher at my school, through observation of a matching game was able to notice that the child was having problems identifying certain colors, while all the other children were able to get it. After discussing this with the parents they took the child to the eye doctor, where they discovered that the student was colorblind. Without the observations, the parents would not have discovered this until several years from now if not at all. This is why observation of special behaviors is important.