The worst thing I have accidentally done to a book was spill tea all over it. I may also have crushed an insect or two between pages and left food-stains inside books.
I have, however, deliberately done worse things to books.
- hollowed them out to make hiding places. To be fair to myself, these were books I bought second hand and started reading, only to discover that there were pages missing. I do have one Reader's Digest book ready for hollowing and I don't feel bad about it at all - it is, after all, a volume of digested, i.e. edited books, and in my opinion that is one of the worst things you can do to a good book (don't, however, get me started on bloated monstrosities like The Historian - that I would gladly edit and it would be better for it).
- given them to my pet parrots to shred (one book with missing pages and one terrible, terrible book that made me mad because of homophobic elements)
- put them in the recycling bin