Several algorithms (std::copy, std::transform, ...) break when an iterator overloads the comma operator.
These algorithms use several help functions where loops are written as below:
for (; _First != _Last; ++_Dest, ++_First)
Hence, "++_Dest, ++_First" might be a function call to an overloaded comma operator (which can be found as a iterator member function or by ADL) rather than the built-in comma operator.
Searching for ", ++" in <algorithm>, <xutility> and <xmemory> yields several places where the issue might appear. (I don't know if they are all.)