When crafting acid rope if the protection scroll that is needed for the recipe is not in your main back pack, and you have a spell book in your pack, it will remove the spell from the spell book.

It took me several very puzzled minutes to realise that the reason I couldn't write any more protection spells to make a second batch of ropes was because my spell book now had only 63 spells instead of its customary 64.

Ooh thanks for that warning! That's a strange bug indeed..

Deffinately good looking out. Would stink to find out you don't have it when you really need it.