Liver Excess Vitamin A
In fresh food feeding, liver is suggested to be fed at around 5% of the diet. Liver is incredibly nutrient-dense and high in Vitamin A which can be toxic to dogs in high amounts or where it has accumulated over time.
It is highly unlikely that a dog will suffer toxicity from a single sitting of liver, i.e. if your dog stole a bunch of liver treats, you're more likely to deal with explosive poos however, we will list below some symptoms that will lead you to call your Vet if you see them!
Vitamin A is fat-soluble, this means it is stored in your dog's body (the fat surrounding the liver) and excess amounts will accumulate over time, therefore if your dog receives an ongoing high amount of liver, your dog could start to develop symptoms of Vitamin toxicity.
"Vitamin A toxicity symptoms may include general malaise, anorexia, nausea, peeling skin, weakness, tremors, convulsions, paralysis, and death".
"More commonly, over-supplementation of vitamin A for weeks to months can eventually result in a poor hair coat, rough or dry skin, weakness, weight loss, constipation, excessive bone development, and painful or limited movement. Excessive vitamin A intake during pregnancy has been associated with cleft palate formation and other fetal abnormalities. 
Elevated levels of vitamin A in the blood will also support the diagnosis. Blood tests to evaluate organ function may also be performed. We recommend all pet parents have annual wellbeing checks with their Vet including bloods regardless of what they are fed - such blood tests can give you some confidence that what you're feeding is on point.
Lots of pet parents use liver treats as training treats, this is not recommended because during training you will be using a lot of treats and too much liver in one sitting is likely to upset your dog's tummy - dried treats also don't make the best training treats, you want something moist and easy to chew like cut-up Prime100 cooked rolls. If you were to regularly do training sessions with liver and also have liver/cod liver oil in your dogs' diet, then that could be problematic over time as vitamin A levels build up.
For dogs, the requirement of vitamin A is 3,333 IU/kg of diet fed, with up to 333,300 IU/kg of diet considered to be safe.
Need more clarification? Speak with your Vet :)