One of the more interesting aspects of the web being so universal is that it gets pulled in a lot of different directions, so sites can stop looking like what you think they look like.
One example is the YAPC::NA/OSDC website, where none of the sponsor logos on the website were working for me, but yet it looked fine for other people in the committee.
The problem turned out to be Ad-Block Plus. But more specifically, because the guy doing the website had naturally enough put the images into an
And of course, since tons of ads are in things with the word "sponsors" in it, all the conference sponsors logos were blocked.
We currently have a similar problem on http://search.cpan.org/ with broken gravatars. Because gravatars are most popular in social network type websites, they fall into the "social networking" category of the corporate fun filter here at work, along with MySpace, FaceBook and all that junk.
Result, ugly broken image square on the search.cpan.org module pages
So note for future, if you ever use gravatars or other similar systems with social networking background for more serious purposes, always make sure to put a squid proxy or some other form of obfuscation on a serious domain in front of the raw service, so that fun filters won't generate false-positives on them.