Why did my substrate turn sour or slimy?

A sour smell and wet, slimy patches usually mean the substrate was too wet or poorly aerated, which lets bacteria take over. Fix it by draining to about 60 to 70% moisture, improving air exchange, and making sure pasteurisation or sterilisation was thorough. Once a block smells sour it is best discarded, because the bacteria outcompete the mycelium.