A Unicode code unit sequence that purports to be in a Unicode encoding form is called ill-formed if and only if it does not follow the specification of that Unicode encoding form.

• Any code unit sequence that would correspond to a code point outside the defined range of Unicode scalar values would, for example, be ill-formed.
• UTF-8 has some strong constraints on the possible byte ranges for leading and trailing bytes. A violation of those constraints would produce a code unit sequence that could not be mapped to a Unicode scalar value, resulting in an ill-formed code unit sequence.