Downsides to using querystrings to version static files?

Question

It seems that in order to force users to refresh cached static files (e.g., .css, .js) some sites (e.g., stackoverflow.com) append querystrings to the end of files. For example:

<link rel="stylesheet" type="text/css" href="http://sstatic.net/stackoverflow/all.css?v=9ea1a272f146">

I think this is a good idea, and am inclined to start doing this as well. However, I have heard that there are downsides, such as proxies not properly caching files if a querystring is present.

What are the downsides to this type of versioning?

Answer 1

Only one downside that I know of: proxy caching (as you said):

http://www.stevesouders.com/blog/2008/08/23/revving-filenames-dont-use-querystring/

http://paulirish.com/2010/announcing-html5-boilerplate/#comment-34976

(I found out about these from the comments in http://www.html5boilerplate.com)

The percentage of users affected by this should be very low, though. I've used this technique for quite a while, and still do.