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

Thanks guys for your comments. I will try to support combining the predicates from Where and First, subject to the constraint that it has to be expressible as a MongoDB query and give the correct results. Keep in mind that other LINQ providers (like LINQ to SQL/EF) also don't support EVERY possible LINQ query, so it's not unreasonable for us to say that some LINQ queries that compile without errors are not supported at run time, it's just a matter of supporting as many as possible.