I've broken lots of things in my life.
When I was a kid I used to break things all the time. Less so now. I've learned to not touch stuff so much. Because, you know, it's the touching that starts it all.
I don't think I've ever broken anything that I didn't touch. Unless you count that time with the boomerang. Or that other time with the kazoo at the museum.
Anyways, I guess you can't fault Captain America for breaking his shield in the last Avenger's trailer. It is, after all, his shield. So of course he can touch it.
And maybe that's not even Captain America's real shield in the movie trailer. Maybe Captain America was fighting the bad guy near a Toys R Us and they blew the place up. You been to a Toys R Us lately? There's Captain America shields everywhere, and they're just made of plastic.
What with product placement in movies these days, I'll bet that's what you actually saw in the trailer. SPOILER ALERT. That probably means Ultron's lair is under a toy store.
Captain America's broken shield in the last Avenger's trailer . . . eh.