Hey Yudansha - you seem like a knowledgeable and cool guy. Like you said - pretty much everything can kill you if you overdose on it. The water one is a good one, as you wouldn't think that ingesting too much water would harm you, but if it dilutes the salts that are naturally present in the body too much, then it can kill you! *Slurp*
Speaking of people who are allergic to water - I read recently that in the long term, taking showers may be bad for you, as breathing in tiny particles of manganese, which is naturally dissolved in the water can damage the central nervous system over time. This might explain the voices in my head which keep telling me to take more showers. Maybe I could design some kind of gas mask for wearing in the shower, which will prevent the manganese from killing me...
GMan