Most pieces take a fair amount of pressure to snap. They usually will get stress marks first and if pressure is continued might finally break. There are some pieces that snap with little effort. Just connecting them to another piece or trying to get them apart will cause the disaster of the picture above.
The 2×4 brick on the left is normal Blue. The one on the right is Brittle Blue. Of the colors that are prone to breaking, Blue is one where there is a noticeable color difference. It looks a little yellower than normal blue though this could also be due to age as well. The easiest way to tell if you can easily break it. So far I have found Brittle Blue, Reddish Brown, Dark Red and recently Green. The color difference in the brittle vs regular colors is not as noticeable as Blue. I just went through a lot of used pieces that at the Brittle Reddish Brown. 50% of those pieces were brittle. It is more than just Bricks and Plates. Minifig torsos and arms easily break if these are brittle colors. A torso will split on the side anyway but the brittle torsos crack everywhere.
As a seller, I try to catch these as much as possible but some could make into buyers hands. I didn’t know about Green being a brittle color until recently. If you open an older sealed set, that set could have these pieces. The Boeing Dreamliner was a set that had a lot of Brittle Blue. As a buyer if you get brittle pieces from any seller, it wasn’t intentional. A lot of sellers probably aren’t aware of this problem unless one breaks on you. These can get separated without them breaking. Most sellers will probably make it right for the buyer but in cases like the brittle colors not everything is the fault of the seller.