Being struck by Permaspinach made me able to test a few maps.
In Level 3 West of the Ilmenskie Caverns, you can walk (and shouldn't be able to) on the bottom left on a distance of about 1 plank. My apologies if this is a secret area where nothing special happens.
http://i.imgur.com/PWPOF.jpg
And now for something completely different: in Getan Tenets, Alakol (aka à la colle), on the top right side, there's an off-limits area that can be reached with a spinach jump, but if the player doesn't have anymore spinach, he can't get out of it by himself (unless he teleports or killer yoga-orders more spinach... but still, the frog might not want to go in that area behind the wall).
Anyway, I let you admire my fine piece of gimp'ed self-explaining picture art:
http://i.imgur.com/OhmBg.jpg