An example would be last year when our museum hosted an exhibition of
Faith Ringgold's story quilts, only one, which was on loan from the
Guggenheim, was displayed under plexiglass. STudents and other visitors
perceived this one to be more valuable (read: better) than the others
precisely because it was shown "under glass." The reason for the display was
simply a contractual stipulation from the Guggenheim--they would only lend it
if it was protected.
I try to include these sorts of points on tours for students in museums
because I think it encourages them to be thoughtful about the biases and
judgements that are "hidden" within our history (not only art history). An
excellent article dealing with this subject was published in a journal a few
years ago. The authors are Carol Duncan and Alan Wallach and the title was
"The Universal Survey Museum." Unfortunately, I don't remember the specific
journal or date.