I've had a few pieces break over the years, which was a real bummer as I was not expecting them to.
What are some commonly used pieces that are prone to breaking? Pieces that should be handled carefully and stored separately.
Lego is generally quite robust, but there are potential weaknesses:
Some parts are prone to damage as a consequence of the material from which they are made:
Damage can also be caused due to the shape and structure of some components:
Long thin parts are prone to bending or snapping under load, for example:
Wheels and gears can crack under stress
Smaller items that fit on technic (axles bushes and the smaller cogs) are easily crushed
Electronic items have special problems:
The ones that break for me are the sockets for the ball joints. See http://www.flickr.com/photos/grandpappy/3988584161/...
The ones that gave me the most trouble were the "new-style" BIONICLE axle connectors like this one:
It seems that the parts holding the axle ball were much thinner than on the previous versions, and they tended to crack and break from extensive use. It was possible to use some cracked ones, but some broke beyond that.
I see no mention of Lego propeller blades. Both when I was a child and now in my kids' LEGO collection I find it hard to get a propeller that doesn't have at least 1 blade broken off.
I've had one or two long technic pins (part 6558) break on me.
This weekend I sorted through some boxes of lego that have laid dormant under my parents' house since I moved out. They contained bricks from the 80s to late 90s. THe only broken pieces I found where these technic chain pieces, crushed at the bottom of the box, and lego traffic signs broken off at the base. I can reuse the traffic signs by sticking them into a 1x1x1 barrel piece, but the chain is ruined and I'll need to buy a whole new one.
I remember that back in the day (which would be mid eighties), I had a number of 1×1 translucent plates split on me. Right through the middle — and then I had two halves. They would typically split either when trying to connect or disconnect them to/from a regular stud. No idea if it was a bad batch or what. Haven't had that happen with more recent versions.