The invention is an apparatus for detecting band overlaps and similar defects on an endless forming surface comprised of a continuous band helically wound in a plurality of side-by-side convolutions. The device comprises two members movably connected together and each being in contact with one of adjacent convolutions of the band. A sensing means, such as an electric switch, is mounted on one of the members and is actuated by movement of the other member in response to an overlap or other radial displacement of the corresponding convolution. The sensing means can be connected to the drive circuit for the band to automatically stop the drive when an overlap occurs.