The nurikabe (ぬりかべ) is a youkai that manifests as a wall that impedes or misdirects walking travelers at night. It is also a binary determination puzzle named after the youkai.

It is impossible to go over or around it as it has the ability to expand itself forever. It is said the only way to get past it is to knock on it's leg or lower part. 

It's known to be more annoying than bad or scary.
nurikabe