1) Place 2 upside-down stairs of any material right next to each other.
2) Place 2 levers on top of placed upside-down stairs.
The levers pop right off. This also works if you try placing 3 upside-down stairs next to each other and placing the levers first on the outer-most two stairs and then place a lever on top of the middle stair. In that case, all three levers would pop off. This was not an issue in Minecraft 1.7.10 and first appeared for me with the snapshots since then and in Minecraft 1.8.
It may be an incomplete change that was made. I'm not sure if you could place levers on upside down stairs before, but what is happening here is the way the game handles blocks that don't belong somewhere, but only after that block is updated. The same thing happens if you use setblock to place a midair lever. if you place a block next to it or activate the lever, it will drop a lever item. Either placing levers on upside down stairs is unintended or the levers are placeable now but being dropped as items was not properly removed/fixed.