On my current playthrough I killed Mr. House and have sided with the NCR. I have done the quest "You'll Know It When It Happens" where you have to defend the president at Hoover Dam. I have not yet returned to Yes Man for "Wild Card: Finishing Touches". I saved the president.

I then started doing more NCR quests. After several quests, in "For the Republic, Part 2", my current objective is "Talk to Ranger Grant about protecting President Kimball during his visit." ... yeah. President Kimball has already visited. This event has already happened. I can't even interact with Ranger Grant. He's just standing there.

What do I do..?

also, when I go to Hoover Dam, it still has the battle music, and everybody is cowering like they're running away. Every once and a while I hear a gunshot. When I talk to the rangers, they just say "It's been such a great honor to be here today" or something like that. The people I could previously talk to I can't even interact with... like not even pickpocket. I also can't wait or fast travel from the area...

I just walked away from Hoover Dam, then waited an hour, then walked back. The crowd of people are now gone, and everything seems to be back to normal (I can wait, fast travel), but the guy I need to talk to is gone... he just disappeared. The quest marker is where he normally stands, but he isn't there... great...I'm going to go talk to Yes Man and see if anything happens.

The same thing happened to me. I posted about it here: http://www.gamesas.com/index.php?/topic/1133049-youll-know-it-when-it-happens/page__p__16590948__fromsearch__1&#entry16590948
but no one responded.

well, apparently, after talking to Yes Man, I was far enough that all I needed to do was talk to him and I could start the last quest... for him, er... me, not the NCR. It was too late to do their stuff.

So I killed all the BoS for nothing... they Idolized me! Now I'll be mentally scarred for life!

I agree, blowing up the entire freaking BoS was incredibly brash, uncalled for, and something no Good aligned character would ever do... and there isn't even a sliver of diplomacy, since they shoot you on sight for some unknown reason. I went through a lot of bloody trouble helping those people, then I just blow them up for the hell of it? No chance of using my Idol status to convince them to help?

I feel even worse now that I can't even get the NCR main quest finished...

oh... and FYI, if you kill the Legate at the end without talking to him first... you can't continue. I had to reload because of this. I was sneaking around the camp and I sniped him before talking to him. My quest marker was on his dead body and I couldn't do anything.

apparently you can't kill him at all in that battle. When you fight the Legate, after he takes a certain amount of damage, he runs away, which spawns more enemies. I chased him down.... and I got flung across the map away from him, then he disappeared. I killed all the enemies, then a scripted event happened.

Apparently if you manage to kill the Legate before this, it breaks the game. This was easy since I had a MkII Securitron and Veronica with a fully repaired Gauss Rifle. He died almost instantly. I kept reloading, not wanting to do the peaceful option to let him get away...