Product Version = NetBeans IDE 8.0 (Build 201403101706)
Operating System = Windows 8 version 6.2 running on amd64
Java; VM; Vendor = 1.8.0
Runtime = Java HotSpot(TM) 64-Bit Server VM 25.0-b70

Consider this class (and imply an import for java.util.function.Supplier):

class MissingCastHintInLambdaExpression {
    void bug() {
        Supplier<String> s = new Supplier<String>() {
            @Override
            public String get() {
                return new Object(); // "incompatible types: Object cannot be converted to String"
            }
        };
    }
}

NetBeans correctly shows a compiler error and provides the hint "Cast ...new Object(...) to String" which results in this (syntactically) correct code:

class MissingCastHintInLambdaExpression {
    void bug() {
        Supplier<String> s = new Supplier<String>() {
            @Override
            public String get() {
                return (String) new Object();
            }
        };
    }
}

When I introduce a lambda expression in the first version, it looks like this:

class MissingCastHintInLambdaExpression {
    void bug() {
        Supplier<String> s = () -> new Object(); // "incompatible types: bad return type in lambda expression", "Object cannot be converted to String"
    }
}

Again, NetBeans correctly shows a compiler error, but it does not provide a hint to insert the cast to String which – once implemented – should change the code like this:

class MissingCastHintInLambdaExpression {
    void bug() {
        Supplier<String> s = () -> (String) new Object();
    }
}