I haven't seen any slugs since I served my seedlings coffee. It made absolutely so sense to me that caffeine would deter slugs, but I tried it. It is possible that it was a coincidence, but it is certainly something I will put on my strawberries as I don't like sharing with slugs.
A lot of things don't make sense to me. I guess that just means I don't know enough to fill in the blanks around it.