At 26:42, Eck can be seen reflected in Dr. Stone's special glasses. At no other time does Eck's reflection appear in the special glasses.
When Eck first encounters Dr. Stone in his office to take his glasses, the blackboard there switches back and forth several times from being entirely covered with formulas to being partially erased on the right side. Actually, there are two blackboards in different parts of the room. Only in the last shot of the story does the camera pan the room to show the separate locations of both blackboards.