Do solar panels ever fail because of shading?

[bfitzgerald]

I purchased 12 - 250w Canadian Solar panels in 2012. All are subject to partial shading from trees every day, until noon, and lots of partial shading in from October through March due to lower sun angles. As far as I can tell, they appear to be working fine but I really don't know if their output performance when shaded is degraded. In short I think that their output performance when NOT shaded has remained the same as when new.