Answer.

The override keyword in Kotlin is needed to explicitly indicate the overriding of methods and properties of a superclass or interface.

Background

In Java, it is possible to override superclass methods without the keyword, which can sometimes lead to errors or typos. In Kotlin, adhering to the principle of safety, it is required to specify override for any overrides and open for the super member itself.

Problem

The risk of accidental method hiding (accidental overriding) and the necessity for explicit control over inherited members. Moreover, overridden methods must be marked as open, otherwise, they cannot be overridden without a compilation error.

Solution

Using the override keyword with methods and properties of a superclass or interface which are previously marked as open, abstract, or already override.

Code example:

open class Animal {
    open fun sound() = "???"
}
class Dog : Animal() {
    override fun sound() = "Woof!"
}

Key features:

Without the override keyword, it is impossible to override the method — there will be a compilation error;
By default, methods in Kotlin are final, and only those explicitly marked as open can be overridden;
The override keyword supports multiple inheritance through interfaces and classes.

Trick questions.

Can a property or method be overridden if it is not marked as open/abstract/override?

No, only members explicitly marked as open/abstract/override can be overridden in a subclass.

Is override mandatory when implementing an interface method?

Yes, always, even if it is the first level of implementation, override is mandatory — that is Kotlin's syntax for consistency.

Can a method marked with override be further overridden?

Yes, if the method is not marked as final (by default, override inherits open), then it can also be further overridden in the hierarchy.

Typical errors and anti-patterns

Forgetting to mark the base method as open — impossible to override, compiler throws an error;
Incorrectly defining intent: a random error in the signature leads to executing the wrong method;
Attempting to override a final method — not possible, compilation error.

Real-life example

Negative case

A developer forgets to mark the base class as open:

class Cat {
    fun meow() = "meow"
}
class Tiger: Cat() {
    override fun meow() = "ROAR" // compilation error
}

Pros:

Simplest class implementation.

Cons:

Cannot override behavior, resulting in an override error.

Positive case

Correctly defining the class and intent for inheritance:

open class Cat {
    open fun meow() = "meow"
}
class Tiger: Cat() {
    override fun meow() = "ROAR"
}