Eating old or spoiled mushrooms can make you sick and it's best to just throw out the mushrooms if they're old to avoid getting sick from them.
Usually what will happen to you when you eat old or bad mushrooms you'll typically just get an upset stomach or have vomiting.
However if the old mushrooms contained the bacteria called Clostridium botulinum and you got infected with that bacteria from eating the old mushrooms then it could lead to serious food poisoning and possible death.
If you've eaten some bad mushrooms then if your stomach ache doesn't go away within a day or 2 then it could be that you got infected with that bacteria and need medical treatment.
If the mushrooms are old it's best to just throw them out and not risk getting sick from them.