In the first Mass Effect, if your Shepard is Earthborn, he/she will at one point run into a member of his/her old gang, the Tenth Street Reds, aboard the Citadel. He's there trying to get another gang member out of turian custody, and asks for your help, so you go into the bar to talk with the turian guard. One of the possible dialog trees:
SHEPARD: You're holding a human prisoner. I want him released.
TURIAN: I want lots of things, human. Desire forms the basis of structured society. That being said, the xenophobe is remaining in custody.
You will eventually learn that what the gang member did was to try to poison medical supplies bound for a turian colony. Assuming you decide at that point to leave him to his fate, the gang member you met outside will come in and threaten to expose your past. Another potential dialog tree!
GANG MEMBER: I bet all your alien friends won't like you so much when they learn you used to run with the Reds!
SHEPARD: I'm a Spectre. I can legally execute everyone in this bar. You think I care what a bunch of aliens think?
TURIAN: Shepard overcame a troubled past to lead a distinguished military career. I can assure you that no turian would think any less of him[her]. And I doubt your lies would impress the salarians or the asari.
One possible dialog tree ends with you shooting the gang member. Afterward, the turian says, "Impressive. Perhaps our human Spectre won't prove to be a disappointment after all."
_________________
Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.