Sour, rotten, or otherwise off smell
Visible mold on the surface or lid
Pink, green, black, or unusual discoloration
Watery separation with an unusual appearance
Thick, lumpy, or curdled texture beyond normal yogurt consistency
Bulging, swollen, or damaged container
Expired date, especially if it has been opened for a long time
Bitter, rancid, or unpleasant taste
Presence of gas, fizzing, or pressure release when opened
Any sign of contamination from a dirty spoon or container