If someone attacks you and you use SELF-DEFENSE, it's definitely not any kind of "injustice"!
One kind of social injustice was when this country practiced "separate but equal" laws and attitudes:
Another possibility is what some have called "reverse discrimination":
(I couldn't find much of anything on this topic that is balanced; just about everything is biased in one direction or another.)
There are many others. Just read newspapers (in print or online: http://www.newspapers.com/ ). You'll find more examples.
To me, and I am a yellow dog liberal, letting the poor go without medical care, or shelter, or food is a grave social injustice. It is a social injustice to let children not have the opportunity for quality education (whether they are or are not now in homeschooling, public, or private schools). It is a social injustice to allow rich peoples income (bonds, stocks) to be taxes at a lower rate than less wealthy folks.
Writeacher posted excellent examples of social injustice in the United States. If you don't live in the U.S., you may want to Google social justice + your country's name. This site gives a broad overview of social justice.