Back in the 1970's when WD-40 was much younger than it is today, there was a rumor that it would cause nickle finishes to go cloudy. In those years it was widely used on guns and many of us who carried daily in humid conditions and wet environments also sprayed the insides our holsters frequently. When that rumor came out I checked my 2.5" nickel M19 and sure enough it slightly cloudy around the high points of the cylinder, so I quit using it. I cannot say there was a definitive connection.

Another problem back then with WD-40 is many LEO's sprayed their guns while loaded at the end of their shifts, and later learned that could deactivate the primers in exposed rounds.