I think about intersectionality a lot with regards to feminism, even if I don't always say so. Intersectionality is basically the study of the intersections between different under-privileged groups. Put simply, a lot of people are seemingly unaware of these intersections, or actively dismiss them while promoting the cause of their own.
(One such intersection is between "black" and "homosexual." Black culture has often been extremely hostile to the issue of homosexuality, to such an extent that many gay black men are driven completely underground. There is an entire subculture of black men who identify as straight, but who secretly have sex with other men, colloquially known as "being on the down low.")
Historically, a lot of very prominent feminists have done a terrible job at intersectionality. Major feminist groups and speakers have said racist and homophobic things, and have enacted racist and homophobic policies. Even today, many old-school (a.k.a. Second Wave) feminists not only ignore the issue of transgender rights, but reject transgender people outright.