Found mostly by accident:

>>> import email.headerregistry
>>> reg = email.headerregistry.HeaderRegistry()
>>> h = reg('Content-Disposition', 'inline; 0*²')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.10/email/headerregistry.py", line 608, in __call__
    return self[name](name, value)
  File "/usr/lib/python3.10/email/headerregistry.py", line 196, in __new__
    cls.parse(value, kwds)
  File "/usr/lib/python3.10/email/headerregistry.py", line 452, in parse
    kwds['parse_tree'] = parse_tree = cls.value_parser(value)
  File "/usr/lib/python3.10/email/_header_value_parser.py", line 2705, in parse_content_disposition_header
    disp_header.append(parse_mime_parameters(value[1:]))
  File "/usr/lib/python3.10/email/_header_value_parser.py", line 2569, in parse_mime_parameters
    token, value = get_parameter(value)
  File "/usr/lib/python3.10/email/_header_value_parser.py", line 2431, in get_parameter
    token, value = get_section(value)
  File "/usr/lib/python3.10/email/_header_value_parser.py", line 2384, in get_section
    section.number = int(digits)
ValueError: invalid literal for int() with base 10: '²'

This probably happens because:

>>> '²'.isdigit()
True
>>> int('²')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '²'