I did some tests when this was released and the results were that the only restriction are "items" which cannot be picked up. So basically, if you manage to place inside your backpack or depot any Object (i.e. anything you see on the map, such as walls, floor tiles, trees, etc), that cannot usually be picked up, you'll have this problem.
So what's with this Dead Human? Well, these specific Dead Humans are the ones which spawn inside a Parcel inside the small Dragon's cave in the Plains of Havoc. These human corpses, which do not rotten and are thus frequently used for decoration, cannot be placed by players inside any container. The thing is, however, that they spawn inside a parcel in the map, and this parcel can be picked up. This is only possible because it's a map spawn and the devs have this kind of freedom, so to speak. When you pick up this parcel and place it in your inventory or depot, you have an un-pickupable object with you.
In summary, this only seems to be a problem with the PoH Dead Humans. Maybe there are some other similar cases of map spawns or rare objects that were changed over the years, but it's not something that will affect the majority of players and shouldn't be of concern.
Note: the tests I made are based on the item sprites CipSoft uploaded to Tibia.com. They uploaded sprites for all items which can be transferred, and we matched these with different filters to reach the conclusion above. This is also likely the reason they introduced this restriction, so they wouldn't have to upload every single Tibia sprite to the website when the majority of them wouldn't be used on the Bazaar.