ClassCastException
at runtime.
Example:
List items = Arrays.asList("string", "string");
List<Integer> numbers = Collections.unmodifiableList(items); // unchecked assignment
The quick-fix tries to generify the containing file, which may expose any problems in the editor and during compilation that previously only appeared at runtime:
List<String> items = Arrays.asList("string", "string");
List<Integer> numbers = Collections.unmodifiableList(items); // incompatible types