Statistically black people are more likely to be arrested and convicted for crimes. So if you use an algorithm to determine who should be arrested and convicted, how long they should be sentenced, and likelihood of recidivism, and seed that algorithm with historical information about arrests and convictions tagged by race (even inadvertently through names or addresses), you end up permanently encoding untrue information like "black people are more likely to commit crimes."
an example from a particular class of crimes:
Study: Whites More Likely to Abuse Drugs Than Blacks
https://healthland.time.com/2011/11/07/study-whites-more-lik...
Black and white Americans use drugs at similar rates. One group is punished more for it.