This error type is reported in Java. It fires when an immutable collection is
returned from a method whose type is mutable.

```java
  public List<String> getSomeList() {
    ImmutableList<String> l = foo(...);
    return l;
  }
```

This can lead to a runtime error if users of `getSomeList` try to modify the
list e.g. by adding elements.

Action: you can change the return type to be immutable, or make a copy of the
collection so that it can be modified.