I can't talk to Cocker Spaniels, but Goldies I can do!
Seeing as you want to know the areas where they go 'wrong.' Goldies are prone to several unpleasant/undesirable behaviours:
Resource guarding - Very often it starts as 'this is mine, don't take it' type puppy behaviour. Usually with lots of excited barking, or running away, or diving toward you then away again. It can look like they're trying to engage you in a game of fetch or chase. People think it's cute and funny. That's their mistake. Goldies are natural retrievers. It's literally in their name. So if a Goldie is bouncing around, refusing to return things - even if you're offering food rewards - then it is a problem. Very often it is the result of poor breeding (i.e. puppies not being carefully attended to in their whelping pen and thus certain puppies not getting enough food or toys - a breeder and littermates should be installing early manners in a puppy) and then misunderstanding from owners. Whilst it might seem sweet that your dog has this twig, and is running away. It's an issue. Because sometimes - not all the time - but sometimes, that dog gets older and starts to guard their toys, or other things, more. They stop barking in excitement and running away and instead stand their ground/growl/snarl etc.
So you need to start young teaching your Goldie to 'leave' and offering them a high-value reward in exchange from day one. Just taking whatever the dog has, or allowing unfettered access to certain toys etc, will just reinforce their desire to not relinquish things. It tends to go from 'playful' (which it's not, but people call it that) to 'guarding' at about 8 months of old. But I've seen this sort of behaviour in very young puppies before. It's particularly problematic if you have children.
Separation Anxiety - Goldies are really prone to this. They naturally want to be with their 'people' and don't like to be left alone. Lots of people call them 'velcro' dogs but that just simplifies their behaviour to unhelpful levels. Goldies need to be trained to be left carefully - you cannot just go cold Turkey and dump them in a crate or a play-pen and expect them to be alright. For a lot of people, the Goldies natural instinct to be with their 'people' isn't really an issue...but it will be for some families. Over the last twenty years or so, almost every Goldie - even the working lines - really has been bred to be a people dog. Most working Goldies I know live in the house, not kennels. If you move too quickly with your Goldie (in part because they are so sensitive - see below) you could end up with a dog you can never leave alone.
Reactivity (Dogs & People) - Goldies are incredibly sensitive, which is great. It's part of why I love them. But it also means if they are not properly socialised (and socialisation is not 'greeting everyone all the time all at once') or experience once to many bad 'experiences' then there is no bouncing back. It won't be excited barking but rather 'stay away from me' type barking - often with lunging and snapping. So it means when you have a young dog like a Goldie - that grows big, quickly, and will often find themselves being barked at by smaller dogs - that you need to take care when exposing it to the world. Lots of people have very nasty dogs off the lead and some dogs are fine being snapped at all the time, Goldie's are not one of those dogs. And the last thing you want is a 40kg dog that's reactive.
Destructiveness/Other Negative Behaviour- There is a reason the KC says that Goldies need two hours of exercise a day. The KC often under-estimates the needs of its dogs, in my experience (it says a cocker needs 'up to' an hour a day, which is just pure insanity). Goldies are intelligent and they are high-energy. Two 30 minute walks a day, and sticking a kong in front of your dog will not cut it. Your dog needs good exercise and to be engaged with - not just have a snuffle matt or a kong shoved at them. There have been so many posters recently with a variety of dogs saying 'but I give my dog a kong twice a day, why is he still barking and stimulation.' Kongs etc. have their place but they are not a quick fix for actual engagement and training. So you need to be prepared to put proper effort into giving your Goldie attention, or it will make it's own mischief - and for some dogs, and people, that's fine. But for lots of Goldies it just means you end up with a 40kg powerhouse with too much energy and not enough training.
You're going to get a tonne of people tell you how their dogs of these breeds are precious princes and princesses and they've never displayed any bad behaviour. Most of the time that's down to sheer, dumb, luck rather than anything they've actually done themselves to develop a happy, healthy, dog.
Ultimately, what many people fail to realise is that a Golden Retriever is not a Labrador. Just as a Cocker Spaniel is not a Cavalier King Charles. The breeds are as distinctive as a Doberman is to a Caucasian Shepherd.