Well yeah it's that buggy snake lake level again .
This one isn't actually game breaking (in fact it's even helpful in some regards), but I am going to post it anyway.
Somewhere to the right of where you get the heart, there is one spot between the giant snake and the black boundary where you can squeeze yourself into and move out of the black boundary like in the screenshot below (you may need multiple tries though)
Then you are free to run around on the outside of the map boundary. By traveling counter clockwise you can eventually reach and enter the originally inaccessible Ink Fort.
And there's even secret inside! In an inaccessible area! So I wonder if this glitch was intended in the beginning.