The following json string cannot be formatted. If one selects it and uses "Format JSON", Notepad++ will hang.
{"k0":[{"k1":"a\"b\"","k2":{"k3":"a\"b\"c"}}]}
There are probably shorter examples. But this basic example works properly :
The same string without the escaped double quotes will format properly to
{
"k0": [{
"k1": "ab",
"k2": {
"k3": "abc"
}
}]
}