I accidentally dropped the lid from a can of diced tomatos in my crockpot of cabbage roll soup! [duplicate]

When you can foods, even in a mass production setting, you're pouring near boiling contents into the cans. So heating it in your soup is no different than having used those tomatoes in the soup. If it wasn't food safe it wouldn't be used for canning. It would do no more harm to eat the soup than it would be to eat the tomatoes out of that can.

Addition:

While there may have been health risk in the past with the coating on many canned products, its incredibly rare in modern day. While trace amount of tin are generally safe, in a larger quantity they can cause nausea and vomiting, but generally no long term effects as these are mostly related to the inability to digest tin. In the UK some canned products are coated in a Bisphenol-A compound which is a known carcinogen. Though this is becoming less common, as BPA is generally avoided in food containment. Though the toxicity of BPA is still widely debated, its generally best to avoid it.

Most cans are made of steel, which if dissolved, even in relatively large quantities is not a health risk. For instance, many pans are made of steel or iron. Most commonly these cans are coated with aluminum as its much cheaper to plate steel with aluminum rather than tin. Aluminum can be toxic in large quantities, but aluminum is very resistant to corrosion, as it tends to form an oxidized coating which prevent most corrosion.

There are some minor safety hazard in consuming the coating on certain canned products, though its increasingly less common to find products canned in these containers that contain potentially hazardous materials. Tin coated cans have made people nauseous in the past, due to the tin dissolving from acidic contents like fruit. Though generally the can has to be rather old, and discoloration is a good indicator in this case to avoid consuming problematic amounts of tin.

I'd say being that there are no reports of people becoming ill from cooking a portion of a can, that its safe to assume that there are no major health implications. Though if you are worried, you may want to contact the company that packages the specific brand of canned goods you are using.

sources:

• Wikipedia, Tin Can
• Mount Sinai Hospital, Aluminum Toxicity

It's nothing. Since you've already fished it out, you're done having to deal with it, just keep going. No harm will come to you or your dish.

Though the other answers correctly state that the lid is harmless, what was dropped into the soup was the lid and additionally everything else on that lid. I had once briefly worked at a warehouse that handled, among other things, some food products. There I learned that the outside of the can is no less important than the inside. I have seen rat poison distributed on a pallet of canned soda, which was later distributed to stores to be put in vending machines.

If the lid was properly cleaned, then there is probably no worry. But if the lid was exposed to chemicals, poisons, or dirt then it would have to be properly cleaned to be safe. I like the "lick test": If you would comfortably lick that lid, then the food is probably safe. If you wouldn't lick the lid, then don't serve the food.