I've hit this issue while playing with tokenize for the pep8.py module.

The tokenize detect_encoding() should report SyntaxError when the encoding is improperly declared.

However it raises a LookupError in some cases.

$ ./python -m tokenize Lib/test/bad_coding2.py 
unexpected error: unknown encoding: utf8-sig
Traceback (most recent call last):
  File "./Lib/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "./Lib/runpy.py", line 75, in _run_code
    exec(code, run_globals)
  File "./Lib/tokenize.py", line 686, in <module>
    main()
  File "./Lib/tokenize.py", line 656, in main
    tokens = list(tokenize(f.readline))
  File "./Lib/tokenize.py", line 489, in _tokenize
    line = line.decode(encoding)
LookupError: unknown encoding: utf8-sig