In my experience, you're right... most situations can be handled politely & easily. I find that most people are simply confused first-timers who aren't trying to be rude, but don't understand that others have sacrificed large amounts of time sitting on a curb to guarantee their spot.
We've seen 2 really ugly incidents in our last 2 trips though.
Last Oct, this guy, his wife & DD stopped right in front of others who had been waiting for Wishes in front of the castle in the Hub area & just squeezed themselves in front. CM's were trying to keep people moving through the hub & then the dad who was already about 7 ft tall, put his DD on his shoulders. To see fireworks. In the sky. People starting loudly asking him to be more considerate & he got immediately irate, foul-mouthed, aggressive & was obviously used to just bullying others to get his way. The CM came over, told him that they were not even IN an area to view fireworks because they were standing in the walkway & to please be considerate to those who had been waiting but the dad just continued his rant louder. There were SO many people trying to get through & the CM's just didn't have the manpower to deal with this one idiot and keep the crowd moving so he got his way. Plus the CM was an elderly gentleman who I am sure didn't feel comfortable getting into it with this aggressive guy & it was only minutes from the fireworks start.
In Dec we tried to take that little path behind Liberty Square that runs down to the boardwalk taking you from there to Thunder/Splash to get through the area while the parade was still going.... it was shoulder-to-shoulder with people & strollers & I mean bad enough that once we got so far in, we could not get out the other side, yet people were still trying to push through. It reminded me of a mosh-pit, no joke. One dad was furious & kept insisting that another lady move her stroller NOW so they could get through & she was like...

Um.... WHERE? She was blocked on all sides & tried to politely explain but he LOST it & started screaming at her, again dropping
F-bombs everywhere. Eventually he & the mom LIFTED the double stroller, with 2 kids inside, over the heads of everyone there & made their way to the queue area for the Riverboat (it was closed already) & exited through their entrance instead.
I don't think a lot of people understand how large the fireworks/parade crowds can be & it's overwhleming to be caught up in the middle of it if you aren't prepared. And if you're claustrophobic, forget it!