Mojira Archive
MC-5850

Buttons, levers, ladders and tripwire hooks can't be placed on sides and bottom of redstone blocks

You can place levers on top of redstone blocks, but not on the sides.
You can't place tripwire hooks, ladders or buttons on the sides of redstone blocks.

Edit: added ladders to the list, updated version (it's still present in 13w09c).