If you eat old mushrooms, you might experience mild to severe digestive problems, including nausea, vomiting, diarrhea, and stomach cramps. In some cases, they can also cause hallucinations, dizziness, and fever. Old mushrooms can harbor bacteria, viruses, and fungi that can cause food poisoning, infections, or allergic reactions. Consuming contaminated or spoiled mushrooms can lead to serious health risks, especially for those with weakened immune systems or underlying medical conditions. Therefore, it’s best to avoid eating mushrooms that look or smell suspicious, and always cook them thoroughly before consuming.