If you are processing UTF-8 and can't handle "forbidden" (for example you are in a language with no exception handling, or this is code which has a must-never-fail constraint) you should treat whatever forbidden sequence you've encountered as U+FFFD ("Replacement Character") and continue. The replacement character is commonly visually represented as a black diamond with a white question mark inside it, so humans will recognise that this means something went wrong, while a machine can't confuse U+FFFD with any letter, digit, white space, or other commonly used element it may be treating specially so this prevents many potential attacks on a parser.

The article is unfortunate because it's actually handling a common non-UTF-8 encoding in which U+0000 is encoded differently so as to keep C-style null-terminated string semantics while allowing the NUL character. You should probably avoid the necessity for such semantics.

The bytes FF FE or FE FF in the beginning of file are often used to indicate whole file is UTF-16 with "byte order mark", not UTF-8. I don't know of any other practical use, it's best to treat it as invalid.