[CoryMathews]

"causing off-flavours or even poisons. "

There is nothing that can grow during fermentation in beer or wine that will hurt you. It will just taste terrible, or unplanned.

In lower abv fermentation such as kombucha or fermented vegetables. Molds can be a risk if the starting medium is not low enough ph, or high enough salinity.

[rwmj]

Something like aspergillus flavus could poison you, although you'd notice that because it would be clearly visible as a green mould with a terrible smell & taste. I homebrew myself and agree that poisoning from ordinary brewing (not distillation) is incredibly unlikely.