mongodb/mongo-csharp-driver — comment on commit b69dc19b15c11cd3fc0d47c0605ad6b32d9cc7e1
File: Driver/Linq/Translators/SelectQuery.cs line 293
By darrenkopp:

The notion of not allowing Where and First both having predicates seems short sighted. The beauty of linq is how composable it is, so often you may be building up an IQueryable in many places and then finally adding some last criteria in the First method when consuming.