I follow this blog http://mightyillusions.blogspot.com for fun. Actually, they have a widget that I use that shows the illusions. You may remember the 3D chalk drawings I posted a while back - they came from there.
Anyway, for no reason in particular here is one that is by no means the coolest but I thought was interesting.
Below is the Big Ten logo. However, since 1990 the Big Ten has actually had 11 teams. They took care of that in the logo although they didn't change the name. There are two 11's in there (hint: one is not in Arabic numerals). Can you see them?