When you place a lever on top of a Hopper and turn it on the lever disappears but the hopper says it is still activated in the F3 menu. Happens in different worlds in different environments also. The Hopper in the picture is filled with 5 stacks of Redstone Dust but still happens when empty.
Environment
AMD A8 processor, 8Gb RAM, latest Java version, game is running on a Kingston 120Gb SSD, Windows 7 Home Premium
I can confirm the lever disappearing, but F3 doesn't show the hopper activated. The hopper still funnels properly.