There is one documented case of a man who consumed large amounts of P. semilaceata and died thereafter. The experts who investigated this case are still not united as to whether it was because of the mushrooms or some other reason.
The LD50 for psilcybin is as follows:
285 mg/kg i.v.(mice)
280 mg/kg i.v. (rats)
12.8 mg/kg i.v. (rabbits)
All of these dosages are intravenous meaning into the vein. To reach these concentrations orally, without vomiting, would take an extreme amount of diligence.
Looking at the scale of comparitive potency and assuming that .6% of an average cubensis mushroom is active (6 milligrams), and that a strong to heavy dose of psilcybin/cin is 20mg, we see that it would take a monumental dose(several pounds of dried mushrooms) to accidently overdose.
This fits the dosage curve (6 milligrams per gram X 4 grams of mushrooms = 24mg dose)