The parser treats separator punctuation (commas between array and object members, colons between object keys and values) as whitespace similar to EDN does. This is not in accordance with any of the JSON specs, in particular the one that the library intends to follow (i.e. http://json.org/).
Some examples:
user> (json/read-str "{,,,\"w\"\"x\"\"y\"\"z\",,,}")
{"w" "x", "y" "z"}
user> (json/read-str "{\"x\"::::\"y\"}")
{"x" "y"}
user> (json/read-str "[1 2 3 4 5]")
[1 2 3 4 5]
user> (json/read-str "[1,,,5]")
[1 5]