It is likely that someone knowledgeable in brewing would not describe the sock as thoroughly or as well as the textile expert would. The photograph of the good secondary source photo of the sock may be an excellent source, if very little data is lost in that generation. Thus one could possibly have a tertiary source that is green-flagged, because it is a good source. For example, an excellent picture of a bad drawing is still a bad source. A careful person should also be aware of the bias of the source, which may present the information in such a way as to invite inaccurate conclusions.
Political campaigns are an obvious example, but bias in art or archeology is more subtle.
For example, if I'm very interested in purple Ruritainian feathers, I may not put any examples of green feathers in my webpage on them. It would be understandable for you to assume there weren't any green ones, but that might be inaccurate. Or half the sock. Such careful analysis can stop the propagation of poor information and replace it with better information. Thanks are due to Kass McGann, who gave this an editorial eye and suggestions, and also her page on good research methods .