From The TV IV
Mouse teeth are an artifact arising from deinterlacing video. They occur when two frames combined for deinterlacing are noticeably different.
This artifact is most noticeable when there is horizontal movement between the two frames, as the lines in the newer frame will be shifted in the direction of motion from the older frame. When the frames are combined a jagged edge (mouse teeth) will be formed.
You can clearly see the "mice teeth" at the bottom of this picture where the scrolling text is located. This is a result of two frames interlaced over each other, and as a result, two fields of motion are visible at the same point in time.