Minecraft wiki entry for Block_update, associated text: Hopper
When being placed, if it should be powered, becomes powered.
Wet sponge
If in the nether, it turns into an dry sponge.
Sponge
When being placed, it absorbs water and turns into an wet sponge.
Target and Observer
When a powered target or observer is placed, it turns into an unpowered state.
Tripwire
When being placed, it updates the signal and connection.
Fire
When being placed, if it should be extinguished, removes itself.
TNT
When being placed, if it should explode, removes itself.
Rail, Detector rail, Activator rail, Powered rail
When being placed, it updates its shape.
Detector rail
When being placed, it checks whether it is pressed and updates its state.
Carved pumpkin
When being placed, if it should turn into a golem, removes itself.