99.99999% of all optic failures are due to not properly maintaining them. It is vitally important that you understand how the optics work and that you regularly inspect, clean, and adjust the optics. PM schedules can be found below.

Here is the most common failure we see.

It takes surprisingly little to disrupt that little beam of light so even a haze on the optic can affect it and the condition continues to deteriorate to the point of critical failure.

It goes something like this... a haze gets on the optic and causes some of the beam energy to be expended on it since it occludes the beam. that area becomes hotter and more debris sticks to it making it darker which absorbs more beam energy. so its thermal runaway. once the crystal gets hot enough the optic delaminates and/or cracks.

Here is a typical example of this anomaly:

It looks as if the crystal may have delaminated in and immediately around the area where the beam is concentrated as, evidenced by the halo around the affected area. This is a typical result of thermal insult.

Here is another example:

And another view of the same optic:

This example suffered from the same condition as the first example just at higher power levels and/or longer time of exposure before the problem manifested itself.

It is vitally important that you understand how the optics work and that you regularly inspect, clean , and adjust the optics.

You can find info on cleaning the optics here: Inspecting and Cleaning Nova Series Optics 

You can find the Preventative Maintenance schedule here: Preventative Maintenance Checklist

as well as in the Unified Owners Manual (just about everything you need to know it in the manual. it should be refenced often): Nova Series Unified Owner’s Manual (US)

ZnSe is a crystal and is synthetically "grown". It is soft and brittle and nothing like glass. Here is how it's made:

And care must be taken not to overtighten ZnSe in its fixture. Deformation will occur. Here is an example of that: