Unknown When `queryable` is of type `IMongoQueryable<T>` the compiler resolves the call to `queryable.First()` to the overload found in `IAsyncCursorSourceExtensions` instead of to `Queryable.First`.
Compiler rules for resolving overloads are rather hard to understand and in this case it resolves to an unexpected overload.
The effect of this is that the query sent to the server does not have the desired `
{ $limit : 1 }` stage.
A similar situation exists for many other extension methods in `IAsyncCursorSourceExtensions` that have the same name as extension methods in `Queryable` or `MongoQueryable`.