Tales of coffee killing slugs are not entirely true, caffine is toxic to them, but they cannot asorb anywhere near the ammount needed from a few coffee grounds on the soil. Caffine is also slow acting, thus a slug exposed to it's fatal level of caffine (by a method much more effective than coffee grounds) still has several hours of life left. Even if those circles of coffee around plants could be fatal, they would still have plenty of time to eat your plants anyway. The actual research that proved caffene killed them openly abmitted the results were minimal, only affected small immature slugs/snails (adults didn't even notice it), and was using a caffine concentration that you couldn't even make from store bought coffee.

It would be amusing, rather like the junk-science idea bottles of water ward off moles and stop annimals fouling in the garden, if it wasn't for the fact coffee grounds when over applied can make soil become acidic and will actually kill the plants it's suposed to protect in some cases. I've met people who put a circle of coffee around chalk soil plants every week all year long, then wonder why the leaves go brown and it looks sick! Of course, slugs actually eat ill plants first where possible (a *real* scientific fact) which causes the coffee wielding plant-killer to douse the unfortunate plant with even more acidic mulch to combat the slugs, who are aparently "eating it so much it's entirely dying!" which only makes their coffee applications more frantic and thicker.

The advocates of coffee grounds even try to pass it off as some kind of old time Granny's remedy, which is total junk as the lab tests which said caffeine could be fatal are not consistent with most old time remedys, in fact I think it was about the 1960's. I've had 50 year olds assure me their grandmother used it to kill slugs, I doubt his victorian era English grandmother drank coffee, and she certainly didn't know about the caffine tests in the 60's, in fact she probably didn't even know what caffeine was...