Visible mold in any color, especially fuzzy spots
Pink, orange, or red streaks or patches
A rotten, putrid, or otherwise foul smell
A slimy, stringy, or unusually viscous texture
Signs of contamination from insects, dirt, or debris
Liquid separation with an off smell after stirring
No bubbling or activity for an extended period, combined with a bad odor
Starter that has dried out completely and developed mold
Any change in appearance that looks unusual or unsafe