Have you ever found these underneath an old oak tree? Look under an ancient Pedunculate oak and you might discover some of these curious looking crinkled lumps. They look really odd and each is different. In a gnarled old oak whilst looking for fungi (spot the fresh brackets of Beefsteak Fungus) I tripped over them in the grass and looking up I could see this tree had dozens of these strange galls. There are Knoffer Galls and they have a wriggley wasp larvae inside and it is the chemicals released by the larvae that cause the normally fertile acorn to go all wobbly.
Funnily enough another species of tree, the Turkey Oak (introduced into Britain in 17350 is need for the lifecycle of the insects and this gall was only found in the UK in the 1960s.