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

What I need to do is add a better error message if you attempt to use the unsupported overload of First and Single. I don't think it's OK to combine a predicate from First or Single with a Where clause. Consider how it might interact with Skip and Take and give different results if the condition is checked before are after Skip and Take are applied.