People who eat porridge/cereal and fail to wash out the bowl immediately afterwards.
As we know, dried porridge/cereal in a bowl is the hardest substance known to mankind.
I will second this, Bubba. The gopher team at my work has to clean up the kitchen and there is always some bastard who has decided to punish me by letting his oatmeal crust in his bowl. I WILL FIND YOU, YOU CRUSTY OFFENDER.