From this article:
Copper is a double-edged sword in brewing. It is beneficial before fermentation, but detrimental afterwards. Copper ions react with the hydrogen sulfide produced during fermentation and reduce it to insoluble copper sulfide, which is left behind with the trub and yeast cake. Switching to all stainless steel brewing equipment can lead to noticeable quantities of hydrogen sulfide and sulfur off-flavors and aromas in the beer. The use of copper wort chillers will provide all the copper necessary, as will including a short piece (1 inch) of copper tubing in the boil.
Copper is a problem post-fermentation because it catalyzes staling reactions, including the production of hydrogen peroxide and can oxidize the alcohols to aldehydes. Finished beer should not be stored in contact with copper, although serving beer with copper tubing in a jockey box should not be a problem, because of the short contact time immediately before serving.