" Wishing Wells are totally unpredictable and generate completely random effects on the party. It makes no difference the day, date, month, or time you use them. These wells can do everything, give bonus skill points, add or subtract gold pieces, remove unnatural aging, as well as give temporary bonuses to stats & resistances. Use them at your own risk but it is highly advisable to save your game first. They can eradicate, kill, stone, curse, cause damage, and insanity. Protection from Magic at GM level will negate the effects of eradication, kill, & stone but NOT damage, insanity or curse.
I have gotten as much as +10 skill points, up to 10,000 gold pieces, & up to +50 temp stat & resistance bonuses. The higher numbers were found more consistently in The Land of The Giants."
This agrees with my experiences.
