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Beam mode

            In this case, we see a massive cross the boards crack, followed by an uncentered burn mark covering the down left area of the CX side of the lens.

            This type of burn is common in a Beam Mode failure; distorted mode can generate a massive energy to be concentrated into a small spot on the lens, leading to a lens explosion.

            In most cases, incorrect beam mode accurse because of mirror misalignment or failure in the beam delivery cooling system.

Mechanical stress

A defective lens mount will generally place a great deal of stress on the edge of an optic.

These stresses will be visible in the form of chipped edges or scratches along the perimeter. Common causes of mount defects include:

-             Metal burrs on machined surfaces

-             Contamination (metal particles) in the mount

Ref: http://www.ophiropt.com/co2-lasers-optics/focusing-lens/knowledge-center/lens-failure?r=drp