The original poster in the thread thought she was seeing something offensive. She raised the issue to get advice. A couple of posters were convinced she was seeing spam and suggested ad blocking software downloads. A number of others had trouble finding the ad and asked more questions. There were several posts about the advertising guidelines that Ravelry uses and how abuses could occur. Five posters found the ad but were sure they were not seeing something offensive and tried to clarify with descriptions. One poster found the ad and did find it offensive. All of this happened in the space of 24 posts and less than 24 hours before the moderators locked down the thread.
The acronym WYSIWYG stands for what you see is what you get. I think it's important to realize that we don't all see the same things in the same way. There are also specific reasons to do with the way our brains work which explain how two people saw something different from the other posters.
"Researcher Mark Changizi of Rensselaer Polytechnic Institute in New York believes it has to do with the human tendency to visually predict the near future. Because there's a lag between the time that light hits the retina and the time when the brain perceives that light, Changizi thinks the human visual system has evolved to compensate for the neural delay by generating images of what will occur one-tenth of a second into the future." (http://www.livescience.com/33664-amazing-optical-illusions-work.html)
BTW: I did email Susan and make sure she has no problems with me writing about this ad. She told me the photo used was in the running to be used as a ravatar for the 2014 Iknitarod but came in second in a vote so it was used for an ad instead.You can find their group here. You can find Susan's site here.