“ExceptionYieldYield” (Solution)

Answer

Call of evenNumbers.FirstOrDefault().

Explanation

The lines

var numbers = GetSmallNumbers();
var evenNumbers = numbers.Select(n => n * 2);

only build a query, but don't execute it. A logic of the GetSmallNumbers() method starts run at the first call of the MoveNext() method, which corresponds to the first call of the MoveNext() method. At this point, the Exception will happen.

Problem