Channeling is when water finds a path through the grounds, leaving some parts over-extracted and others untouched. You taste it as a mix of bitter and sour in the same cup. It's usually caused by pouring too hard in one spot or an uneven bed of grounds.

To fix it, pour in smaller, controlled circles, starting from the center and moving outward. Make sure your coffee bed is level before you start pouring—tap the brewer gently to settle it.