Keith's Caravan Charade

Keith's Caravan Charade is an unmarked quest

Detailed walkthrough
Offer Captain Parker a helping hand. He asks the Courier to find evidence that Keith is involved in chem dealing and crooked gambling, adding the note "Investigate Keith's gambling and drug operations" to their Pip-Boy. Locate Keith's deck of marked cards in a desk in the southwest corner of Aerotech suite 200.

One can discuss the deck directly with Keith by passing a Barter check of 45, which allows them to purchase Jet from him, and a Speech check of 60, in which he lets them in on his 'system' for winning at caravan. This does not affect the outcome of the quest.

Once the Courier returns to Captain Parker with their findings, he rewards them with 150 caps and the option of assisting him in arresting Keith for an extra reward. The captain then enters Aerotech suite 200 and confronts Keith, who taunts Parker with slights about his ex-wife. Enraged, Captain Parker murders Keith. Talking to Parker afterward nets an additional 100 caps.

Bugs

 * Rex may turn hostile against the junkies, causing Keith to flee and ending the dialogue between Keith and Captain Parker. This may be related to the bug that triggers seemingly random attacks on Rex. To fix, reload from an earlier save and take all of the food and water items out of Rex's inventory.
 * Captain Parker may disappear from the game when entering Aerotech suite 200 to arrest Keith. This can be fixed by reloading a saved game.
 * If Captain Parker's quest The Coyotes is in progress and has not been completed, it may not be possible to show him Keith's marked cards. The only dialogue option is to mention that you're still looking for evidence.
 * Sometimes when Captain Parker goes to confront Keith about the marked cards, Parker will not initiate the conversation with Keith but leave as if he did. This makes it impossible to complete any of the unmarked quest. Captain Parker's dialogue will stay as if he was never shown any evidence of Keith's wrongdoing but will never give the option to show the evidence.
 * After completing the quest and Captain Parker kills Keith and gives a reward, the option to speak to Captain Parker about Keith again may appear. Parker will give the information on Keith again as if Keith were still alive. Needless to say, the quest cannot be repeated.
 * Captain Parker might shoot Keith again after giving the extra caps for helping confront him. If this occurs, talk to Captain Parker repeatedly and say goodbye to watch him continue to shoot the dead Keith.
 * If Keith is sleeping when Parker confronts him, Keith will get up and then go back to sleep as soon as the conversation is over. Captain Parker will spin around in circles with his gun drawn above the sleeping Keith. This is corrected by talking to Keith again. As soon as Keith gets up off his bed, Captain Parker will kill him.