Plant physiology is considerably different to that of humans and substances that are toxic to humans often have negligible effects in vascular plants.
Toxic substances can indeed be present in the extracellular and intracellular fluids in plants, and yes, it has led to fatalities.Whilst the toxic substances are usually taken in via the roots, there have been well-documentated cases of absorbtion via leaves or plant stems. The predominant reason we are told to wash fruit is because it's likely to contain tracesof pesticides used during cultivation. Washing is obviously ineffective if the juice is contaminated.
Now it would be foolhardy of me, or anyone else, to disclose even one of these substances for the reason implied by ChuckFickens. Whilst I have no intention of accusing you of wanting the answer for dubious purposes, this is an open forum and it might stir some malicious thought in someone less innocent who reads a detailed answer.
ChuckFickens is not a dimwit - it's a valid point whether you like it or not. The dimwit is the person who spells out the names of these substances and you should be able to see that.