When a lever is placed on any type of slab and flipped on/off it will break dropping a lever, in addition placing a lever side by side (on slabs) will cause them to break. Hope I can help out by issuing this bug, its quite the odd one.
Zi-Wen Lin has pointed out this also happens when a lever is placed on a hopper and switched on.
Pictures are to show before and after shots of turning on the lever, and placing them side by side on slabs.
Environment
Mac OS Mavericks, Java 7
The same thing when you place a lever on a hopper.