Examples of informal amendments to the United States Constitution are when rights were changed from land holders to all men, then to anyone age 21 or older and finally to anyone over the age of 18. An informal amendment can be used by any branch of the government.
An informal amendment is often used when changes to an existing amendment need to be made if existing amendments need to be interpreted differently.
Other examples of informal amendments include laws allowing gay marriage and laws that forbid whites to marry blacks. Each of these starts off as an informal amendment and, with time, are added as a formal amendment to the Constitution.