Unit testing complex LINQ statements
LINQ is amongst the greatest features of c# and when it was released people instantly took to using it. It forms the basis of may ORMs and other object manipulation tools. It can form a great abstraction when loading and manipulating data. It can however like a lot of great tools be horrendously misused by a lot of people. I have seen 10+ line LINQ statements dropped into the middle of large blocks of business logic.