Standardised languages often have "grey areas" - features or (combinations of) conditions for which no behaviour was defined. Any implementation of the standard may react however it sees fit when it encounters such a condition, either because implementors were explicitly granted such freedom by the standardisation committee, but many times simply out of necessity because this condition was overlooked (or no attention paid to).
Examples of UndefinedSemantics include
Perl is amply documented, and pretty much every obvious feature's behaviour is explicitly guaranteed by the documentation. So no, it doesn't "consist entirely of UndefinedSemantics" by a long stretch. --AristotlePagaltzis
One page links to UndefinedSemantics: