PCPlayStation 3Xbox 360 Carlitos and Joana won't appear in the game, if you have advanced far enough in the quest How Little We Know even after you started conversation with both, making the quest impossible to complete.
To fix this on PC, use the following console commands: For Carlitos: prid 0011f87a, resurrect, enable. For Joana: prid 112471, enable, moveto player while standing in the courtyard.
The downside of this method is that once you have to escort her and the disguised hookers out of Gomorrah, they may not move. A solution to this is to start combat with Joana, leave Gomorrah, then go back in and restart dialogue with her. After that, exit Gomorrah, enter Freeside, and Joana and the disguised hookers should appear next to you resuming the quest.
PCXbox 360 Once the group reaches Freeside, the thugs that are supposed to interrupt the escape, sometimes won't appear. This can be bypassed by using the console command setstage 00110a65 90 and talking first to Carlitos once (before or after the command), then to Joana.
PlayStation 3Xbox 360 After escorting Joana and the disguised hookers to Freeside, Carlitos might not appear making the quest incompletable when Joana, the disguised hookers, and the Omerta thugs stand around and do nothing. Killing the Omerta thugs won't work since Joana and the disguised hookers still need Carlitos to appear to advance the quest dialogue. Reloading the auto-save from when you entered Freeside can fix this.
PC When leading Joana and the disguised hookers outside the gate to Freeside, the Securitrons posted outside the gate can become hostile, attacking all included people. Killing them gives infamy for the Strip but letting them kill Joana fails the quest.
Xbox 360 After running back and forth a few times the dialogue option to get Joana to run away might fail to appear; you will need to have a high enough speech skill to convince her.