I was wondering, moonshine is said to cause blindness from a methanol reaction to formaldehyde.when does this?

when does this occur? wouldn't one just go about distilling the corn mash at the tempeture that ethanol evaporates ? Ive read places that if done wrong , one may create fermaldyhyde during the distillation, i dont see how maybe one can explain. Also, i know that in prison , people make alcohol with bread and sugar , then filter it through shirts ( iam uming this is just one of many ways) . Yet this method can also cause blindness from fermaldyhyde. So then why isn't fermadyhyde formed in beer since it is not distilled. so all in all i would like to know where the fermaldyhyde is formed during the production of alcohol