This is what happened last friday on a big cross race:
(For those who don't follow cx, the Belgian Champion you see is Sven Nys, a living legend, maybe the best ever)
But of course there's tons of examples of road races too.
Leif Hoste in Roubaix 2004
Guerini tdf 1999
I took this 'older' examples to show it's not a new thing.
But what is definitely getting worse, is the crowd on the mountain stages.
Compilation vid: http://www.youtube.com/watch?v=oQgWWceYwbY
I it's safe to say this is a problem in cycling. But how to deal with it? In football they can ban supporters from the stadium. The can give huge fines to streakers, etc. Also, they have the possibility to put stewards and police every 2 meters in the stadium. Road cycling is raced on public roads, over more than 200 kilometers.
And I think even if it would be possible to put a police officer every 3 meters on the course (which it isn't), I don't think doing that would be a good evolution for the sport, and the viewing experience.
But what else is there? How can we educate the fans?