You'll Know It When It Happens

You'll Know It When It Happens is a main quest in Fallout: New Vegas.

Synopsis
Aaron Kimball, President of the New California Republic is visiting Hoover Dam to improve morale among his troops. Naturally, the NCR seeks to keep their President safe while he's in a war zone and their political leader, but Mr. House wants Kimball alive for a different reason: he posits that as long as Kimball remains alive, he remains a scapegoat for House, meaning the NCR public will blame him, and not New Vegas, for the NCR defeat at Hoover Dam.

Walkthrough

 * 1) Speak to NCR Ranger Grant at Hoover Dam to make preparations for Kimball's arrival.
 * 2) Conduct a security sweep throughout the dam
 * 3) Talk to Allison Valentine on the second floor of the visitor center. Her engineer friend has gone missing; a blood stain can be found in the supply closet on the first floor and requires a Perception of 6 to notice.
 * 4) Hack the computer terminal in the visitor center (Science 50 required) to reveal that the terminal was accessed with authorization.
 * 5) If Craig Boone a companion, he will remark that if he were a sniper, he would shoot from the ridge left of the road heading north, the intake tower behind the stage or the roof of the visitor's center.
 * 6) Once you have conducted your search, return to Ranger Grant.
 * 7) Prevent the three different assassination attemps:
 * 8) A sniper, dressed as an NCR Ranger, will appear atop the intake tower behind the stage set up for the speech;
 * 9) A bomb is placed on Bear Force One by an engineer;
 * 10) A Legion soldier, disguised as an engineer, will run up to the President and try to assassinate him with a knife. If Ranger Grant is told of either of the two attempts detailed above, this is avoided.
 * 11) Report your findings to Ranger Grant.

Behind the scenes

 * After killing the Legion sniper on the guard post and using the radio to report to Grant, one of the dialog options ("We're all fine here, thank you. How are you?" ) is a cultural reference to dialogue said by Han Solo from Star Wars Episode IV: A New Hope when he and Luke Skywalker are saving Princess Leia from the Death Star.
 * A majority of this quest was implemented by Charlie Staples.

Bugs

 * If you start Wild Card: Finishing Touch after starting this quest, you won't be able to talk to Ranger Grant (only one dialog option - Goodbye) and thus unable to start the event. Your only option is to finish Finishing Touch and let the President die.
 * This bug can be fixed in the console. Select Ranger Grant in the console or type and hit enter. Then type  hit enter, and type . He will be reset and start talking to your character. It may be required to disable and wait several hours then re-enter  and.
 * If you approach him from behind also seems to fix this bug, as it makes him start the conversation.
 * After killing the sniper, report back to Ranger Grant. His first dialogue will be normal but his second sounds like it's coming through an intercom.
 * You can automatically fail the quest if you take the ladder down to the shore and swim to the opposite end of the Hoover Dam.
 * If you disarm the bomb on the vertibird and then immediately run to the tower where the sniper spawns, climbing the ladder to the top of the tower may initiate the following sequence. When you reach the top, you will hear the scream of someone falling, but the Ranger sniper is still there alone and fine. If you wait through the speech on the tower with the Ranger sniper, nothing happens until the very end of the speech where a shot will ring out, President Kimball will disappear, the Ranger sniper on the tower (next to you) will disappear, and you will fail the quest (the only dialogue option when talking to Colonel Moore afterward is to tell her that the President was killed by a sniper).
 * It is possible that Kimball's Vertibird may land and then suddenly take off. After this, the quest will be rendered complete without you having to stop any assassination attempts.
 * When the Vertibird is flying to come over to the base to land. Press X and you'll complete the quest without having to protect him.
 * Sometimes, even though you start the quest by speaking with Ranger Grant, after you investigate the closet and check the terminal, going up onto the observation deck will not trigger the Vertibird landing and Ranger Grant will not initiate dialog any more.
 * After initiating the dialog where Ranger Grant tells you to get some rest before the President arrives, the game may crash after fading out.
 * Though there are three assassination attempts, not all of them may be triggered during the quest. The bomb on the Vertibird will usually go off unless you deal with it, but the sniper and assassin in the crowd may not act.
 * If President Kimball finishes his speech and successfully makes his way back up to his vertibird, he may start walking endlessly against the ledge of the roof of the visitor center away from the vertibird, and never leave. To fix this, talk to Ranger Grant regarding the bomb or the sniper, and this will initiate the escape sequence for President Kimball, forcing him to depart in his vertibird.
 * When you try to pickpocket the engineer and take his detonator and keys, without disabling the bomb in the President's vertibird, it still would explode. even after you protect the President from the sniper and the assassin. the trick of this glitch is to kill the engineer right after the vertibird landed, causing the President to flee safely before he is able to start his speech.
 * When you start the quest talk to ranger grant outside tell him you are ready then run up to the tower behind the stage climb the ladder then climb down and it should say quest completed.
 * It is possible that when climbing the ladder to the tower with the sniper assassin while having ED-E as a companion, that all NCR troops will become hostile to ED-E. Easily fixed by telling ED-E to go back to the Lucky 38 with the latest patch since it will despawn him on the spot. This also forced the rangers to evacuate the President and complete the mission.