The following command, when referencing a skull block entity, causes an error message to be produced:

/data modify block <x> <y> <z> custom_name set value "0"

The error is "An unexpected error occurred trying to execute that command", with the tooltip being "Not a JSON object: 0; Not a json array: 0; Not a string: 0".

I would expect no error message to be thrown, and the modification simply to fail, as with what happens with other block entities that can have custom names.

This issue seems to be caused by the method introduced in 1.20.6-rc1 not being used to parse skulls' custom names (developer ran a search on CustomName and not custom_name, perhaps?)