Minecraft wiki entry for Block_update_detector, associated text: Placement of blocks
Destruction of blocks
Water or lava level changes
Freezing of water or melting of ice and frosted ice
A gravity-affected block (e.g. a sand block) landing after falling
Movement of blocks by pistons, including piston heads
Endermen picking up and putting down blocks
Saplings growing into trees
Mushrooms growing into huge mushrooms
Growth and spread of plants, including: cacti, flowers, grass, mushrooms, melons, pumpkins, and sugar canes, cocoa beans, bamboo, crops (e.g. melon stems), nether wart, and vines)
Leaf growth and decay
Growth and death of grass blocks
Dirt becoming farmland, farmland becoming hydrated, farmland reverting to dirt
Fire igniting or burning out
A furnace beginning or ceasing to smelt
A redstone ore block beginning or ceasing to glow
Changes in redstone state or power level, and activation of redstone devices
Changing the delay of a repeater
Rails changing orientation
Powered rails and activator rails getting powered or depowered
TNT igniting and becoming an entity
Silverfish entering a block
Cake being eaten
The opening or closing of a chest or trapped chest
The activation of a tripwire (even if not attached to a tripwire hook).
Vines, crops (e.g. melon stems), nether wart, or cocoa beans growing
The upper block of a 2-block tall plant (e.g. tall grass, rose bush) replacing/being replaced by an air block when that plant is placed or broken
A dragon egg replacing an air block when it teleports to a location after a player tries to break it
A grass block changing to dirt due to sheep "eating" the grass
A chorus flower growing into a chorus plant
A dry sponge absorbing water
Switching a comparator between comparison and subtraction modes
Playing a note block
Changing the number of layers in a snow layer block
Leaf distance from a log changing
Scaffolding distance changing
Composter fill level changing