Slimy or sticky surface
Strong sour, fishy, or ammonia-like smell
Dark, wet, or moldy spots
Wrinkled, shriveled, or dried-out texture
Soft, mushy, or collapsing flesh
Discoloration that looks unusual for the type of mushroom
Visible mold or fuzzy growth
Caps or stems that are broken down or leaking
Packaging that is swollen, damaged, or leaking
Mushrooms past the expiration or use-by date
Off taste after cooking, if already checked and safe to taste
Any sign of spoilage combined with uncertainty about freshness