We've all pulled at least one or two.
Today, I'm making decaffienated coffee. And I'm going to keep doing it until April Fools' Day whereupon I shall unload caffiene onto them again.
Most will notice the difference, yes, but I'm hoping some won't care enough to get up and remake a whole new pot. Because they're usually too lazy to make a new pot after they empty one anyway.
Got any pranks that you pull regularily or are you planning one for April Fools'?
EDIT: I stand corrected. No one's noticed yet - not even my team lead. I've made two new pots since the first.