This quest completes or fails in tandem with You'll Know It When It Happens. Protecting President Kimball requires that either three assassination attempts be stopped, or he is forced to end his speech early. Once either objective is met and House is informed, the quest will be complete.
ps3 At times, while the quest is active if you choose to equip NCR armor that causes the faction glitch to happen prior to speaking to Ranger Grant, you will gain NCR infamy and the entire Dam (including Grant) will attack you. Even after killing Grant, changing clothing, or waiting any amount of time it will be impossible to complete the quest. When returning to Mr. House he will continue to tell you to go back to the Dam and protect the president, even though the president will now never show up.
ps3 Sometimes when trying to complete Part VI and protect President Kimball, President Kimball stays at the microphone, even after the speech was cut short. This results in not being able to finish the quest. Standing on the platform near the president does trigger the running animation but the president stays on his place. Reloading the save doesn't resolve this problem.
xbox360 Sometimes, if your reputation with the NCR is too low to complete the "You'll Know It When It Happens" Quest (aka The House Always Wins, Part VI), Mr. House will fail to recognize this and send you to Hoover Dam anyway. Upon arriving at Hoover Dam, you will fail "You'll Know It When It Happens," but the active goal for "The House Always Wins" will still direct you to Hoover Dam and prompt you to search for Ranger Grant. Furthermore, your reputation with the NCR will drop, and the soldiers at Hoover Dam may become hostile. Upon speaking to Mr. House again, he will still demand you go to Hoover Dam to save President Kimball. Reloading a save prior to completing "The House Always Wins, Part V," may cause Mr. House to immediately recognized your NCR reputation is too low and start Part VII of the quest instead.