Property damage that results from a continuous and ongoing process over an extended period of time rather than from a defined, identifiable event such as a tornado or hail damage creates disagreements and disputes as to when property damage "occurred."  Coverage under most insurance policies is triggered by an "occurrence."  As Dana Ferestien commented, insurers and insureds continue to litigate over whether or not particular damages are an occurrence under a particular insurance policy. 

These coverage disputes become even more complicated when there are multiple insurers during the period of time in which the damages occur.  Not only is there disagreement over the time period in which the damage occurred, the policies may have different policy language resulting in different outcomes.  The "occurrence" issue arises in homeowners’ policies as well as commercial properties.  Since the damages often go unnoticed for long periods of time, the damages often turn out to be significant in terms of the dollar amount for repair/replacement.  The questions over the cause for the damage often results in both sides hiring experts who have differing opinions. 

I agree with Ferestien the "occurrence" litigation will likely continue as policyholders and insurers assert their competing and opposing positions.  In Oklahoma, attorneys familiar with insurance litigation over coverage routinely defend claims asserting there was no "occurrence" under the terms of the insurance policy and will likely continue to do so.