Sour, fermented, or off smell
Slimy or mushy texture
Mold on the rind or flesh
Dark, sunken, or wet spots
Flesh that is brown, gray, or unusually discolored
Fizzy or bubbly juice
Bitter or unpleasant taste
Cracked, leaking, or heavily damaged rind
Soft spots throughout the melon
Seeds or flesh that look unusually dried out or shriveled