https://blog.greendealflow.com/the-five-most-common-problems-with-solar-panels said:
1) Hot spots
Hot spots are places on the panels which are overloaded and therefore become warm. Hotspots on panels are mainly caused by badly-soldered connections, or are a result of a structural defect in the solar cells.
Badly-soldered connections cause low resistance in the part of the panel that receives the power generated by the cell. As a result, the voltage can rise, which leads to a hotspot in the soldered points and/or a cell. This phenomenon can ultimately lead to a short-circuit, and reduces the performance and lifespan of the PV panel. We have identified hot spots in several solar parks, leading to solar panels being replaced, adding significant Opex costs to those projects.
4) Snail trail contamination
Snail trail is a discolouration of the panel which usually only manifests itself after a couple of years of production. Snail trails have multiple causes, but one cause can be attributed to the use of defective front metallization silver paste, in the solar cell manufacturing process. Defective silver paste can lead to moisture in the panel, and as a result of this moisture, an oxydation can occur between the silver paste and the encapsulation material called EVA (ethylene vinyl acetate).
This unwanted process releases silver oxide, acetic acid (vinegar) and hydrogen. The effect of this reaction is fed from the back of the panel to the front of the panel, and causes a chemical breakdown on the front of the panel. This becomes visible as snail trails, resulting in a reduction in the panels performance. The snail trails can also arise as a result of microscopic cracks in the panel.