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.
The same thing when you place a lever on a hopper.