If the grain mash is left fermenting to long it can only produce
methanol which is wood alcohol if inferior yeast start to process the
sugar. This can only happen if there is sugar left after the
distillers yeast have finished / died off.
The grain mash can change to vinegar if the mash comes into contact
with a large amount of air.
All you have to do to stop both of these things happening is stop the
mash having contact with the outside air by using an air lock or a