The most simple “mechanical failure” you may find is theft of equipment.
Solar modules can be a valuable commodity, as can wire, batteries, controls, and other equipment.
On older roof-mounted systems, you may find sealants have degraded, proper roof flashing may not be present, or racking may be constructed in such a way that leaves or other organic materials obstruct the flow of rainwater.
Concrete may be cracked, or if concrete piers are used, water flows may have eroded supporting soil.
Steel may be rusting, if preservative paints have not been regularly applied.
It’s also possible for the module internal electrical connections to fail, leading to loose or open connection.
This can also cause burn marks between the cells in the module, and the glass on the front of the module.
For this reason, use of stainless steel racking hardware is highly encouraged.
It is also important to ensure that the screws nuts are properly tightened.
There should be no cracking, cuts, or other damage.
Also inspect the outside of the module junction box, making sure there are no bulges, cracking, or other defects.
Any solar electric system may have a number of potential hazards present.
Before working on these systems, each person should have proper training to recognize these hazards, have proper equipment to protect against such hazards, and have a safety plan.
Any evidence of burning means the module should be replaced, to remove any danger of fire.