Continuity mistake: The sequence in which Dragon is chasing Mrs. Brisby through the field, up the tree and eventually into the mill's lake, watch his eyes. The original placement is (from viewer's perspective) blue on the right and gold on the left. When he chases her up the tree the gold one is visible on the right, it then switches back when the camera cuts back to a wide shot. This happens again a few seconds later near the lake.
Continuity mistake: At one point during "Flying Dreams" when Timothy is being cared for by Mrs. Brisby and the other children gather at his bedside, his left arm appears in various positions as the shots change - instantly on top of and then beneath the quilt. (00:18:35)
Continuity mistake: Upon his first encounter with Mrs. Brisby, Jeremy is tangled up in a mess of red string, including his left and right ankles. The string around his left ankle somehow vanishes for an entire shot, but reappears two shots later when Mrs. Brisby warns of Dragon. (00:08:05)
Continuity mistake: While searching for an entrance in the rose bush, Mrs. Brisby grabs Jeremy by the beak and tells him to be quiet or else. In this scene, Mrs. Brisby's right arm changes position. At first it's wrapped around Jeremy's beak, but then suddenly it appears on top in between shots. (00:32:25)
Continuity mistake: While being tied up by Auntie Shrew in the field, a length of string wraps itself around Jeremy's beak during one shot. (00:45:05)
Continuity mistake: When Mr. Ages is within the thresher's shaft suspended in a basket, Mrs. Brisby goes on to tell him that her son Timothy is very sick. Notice how her hands are positioned right over left. Once the shot changes, her hands instantly switch places. (00:04:45)
Continuity mistake: In the first shot of the children and Auntie Shrew washing clothes, one of the bubbles in the bottom right-hand corner of the screen starts out blue, but then flashes red.
Continuity mistake: The Brisby home has a round window on the side of it. But when the rats are moving it near the end of the film, there is no window at all.
Continuity mistake: When Nicodemus is telling Mrs. Brisby that he looked at the words under the latch on the door and understood them, there is a clear, large label under the door. Then a different angle is shown and there is no label on any of the doors, but you should be able to see it still. (00:52:25)
Continuity mistake: When Nicodemus gives Mrs. Brisby the red amulet, she notices an inscription on the back of it and the camera zooms in to it. Before that, there was no inscription, and the back was blank.
Continuity mistake: While Mrs. Fitzgibbons is in the bedroom talking to her husband about NIMH, she pulls a window shade all the way down. But from the next view, where several rats are transporting an extension cord across the front yard's walkway, the shade is back to its original position again - about halfway up. (00:19:55)
Answer: Mrs. Brisby tells Nicodemus that Jonathan had been teaching her to read and her children as well. The children were better at reading that she was but that might be something that they inherited from their father. I think Mr. Ages was also able to read so that makes him intelligent as well. Remember the injections that the mice and rats were given also gave them the ability to read.