[HelloNurse]

The "surprise" in the article is related to a very general class of mistake: supporting type something& but not also type something&& despite that parameter type being decisive for overload resolution purposes and despite the missing function being identical to the existing one.

Overloading the comma operator doesn't cause any particular danger, it merely provides a mildly unfavourable situation in which an unnecessary mistake is possible: there's "threat" of the default comma operator that does something completely different, it's not very obvious whether type something or something& or something&& is considered, and the involved overload resolution rules are somewhat lawyer-grade.