When riding a pig with carrot on stick, other pigs follow you too.

Sometimes sound effects get skipped (e.g. Creeper hiss)

Baby chickens unaffected by flowing water.

Blocks lighting up when breaking them, graphical glitch.

Lines between all blocks (pink, white, and black) / Custom graphics driver settings

Mobs showing shaky movement after hitting them (as if they're hesitant)

Slimes having choppy movement

Weapon/tool durability decreases when hitting playing in no-pvp mode. (SMP)

Cave sounds being cut off.

Chest glitch that lets them appear as if they are opened while they are not.

Squids spawning everywhere.

Two zombies pushing eachother away when attempting to break (villager) wooden doors.