Unity Sample Code :
rigidbody2D.velocity.y = speed;
The above C# code won't show any syntax error. But unity 4 and above will throw runtime error saying
"Cannot modify a value type return value of `UnityEngine.Rigidbody2D.velocity'. Consider storing the value in a temporary variable"
Since we are trying to modify the velocity of the object without assign the object into a variable.
The below code will run as expected.
Vector2 vel = rigidbody2D.velocity;
vel.y = speed;
rigidbody2D.velocity = vel;
Similiar coding style when writing coding in javascript on Unity 4.
