Overstocking are basicly a recommendation in the same way you're recommended to not consume more than 400mg caffine per day. It is possible to do just not good for you. Overstocking on fish has several bad effects on them but everyday you still see people doing it and it turns out "fine".

The main problem is that living in a crowded aquarium will stress the fish and lessen it's lifetime And immune system. And with some fish outgrow the aquarium. But short time you won't notice anything, this is usually why some people claim that their fish is "doing fine" in a way to small aquarium in reality the fish is not fine but unless it gets sick it's hard to spot a shorter lifetime and a weak immune system.

The other problem is a increased waste production. You probably know about this and since I'm on mobile I'll hop this part (I'll explain it if you want though) However you can bypass it by having a sump, increasing your water volume. For example using a 5gal with a 10 gal sump you get the water volume of a 15 gallon tank making just the space in the 5gal the main problem with overstocking.

So yes you can overstock a aquarium and if you avoid a ammonia spike you'll be fine until one of them gets sick and it spreads to all your others leading to mass death but if you avoid that you have a good looking aquarium until they just die in advance because of the stress. Unless you know what you're doing of course