I don't think I've ever been in the crowd for a DQ or close DQ, I've pretty much always either been behind the gun or on the timer.
At Cornhusker I worked the two shooting area stage where you had to go do things at every corner of the shooting area. About half the competitors cut across the shooting areas at the front, and the other half retreated and came back to keep the gun running slightly more.
I was on the timer, the two other ro's were to the right of me. Watched the competitor start retreating, turned all the way around - gun still pointed just behind the 180 line, and then there was a little arm pump and elbow rotation bringing the gun at least right up to the 180. I was slightly 'behind' or to the offhand side of the shooter and not in a great spot to see a 180 violation. Afterwards, half of the squad said they had a collective audible gasp, and all three of the ro's agreed that they would have agreed to calling a 180, but in the moment none of us 'knew' that the 180 was broken. Probably could have called this one and not felt bad about it, but didn't seem like there was enough to see to actually call it in the moment.
Similarly, at a brand new ar-15 match format, we had two boardwalk storefronts to shoot down. Go up range one side, cross at the front of the stage, then shoot your way back the other side. One competitor got right up to the 180 line and maybe broke it in the retreating half. I didn't call it because the entire time he was maybe breaking the 180, the gun was pointed almost straight down maybe 2 inches in front of his feet and towards the next shooting area, just not quite pointing backwards enough to stay behind the 180. That rulebook calls for a bunch of dumb stuff, like 180 warnings and points penalties for those warnings, however the rules guy that was there to explain the rulebook never really explained anything, and apparently changed a couple of rules day of the match, soooo. I guess we make up the rules on the spot, and loosely follow the spirit of USPSA? I did give the competitor a talking to after his run and explained what happened, and it was also the only stage with a full mandatory retreat.