A non-standard Javadoc block tag was used.

/**
 * @returns two times n
 */
int twoTimes(int n) {
  return 2 * n;
}

/**
 * @return two times n
 */
int twoTimes(int n) {
  return 2 * n;
}

Note that any Javadoc line starting with @, even embedded inside <pre> is interpreted as a block tag by the Javadoc parser. As such, if you wish your Javadoc to include a code block containing an annotation, you should surround the snippet with {@code ...}. Alternatively, and if you are using a version of javadoc prior to JDK 15, you may escape the symbol using {@literal @}

/**
 * Designed to be overridden, such as:
 *
 * <pre>{@code
 * class Foo {
 *   @Override public String toString() {return "";}
 * }
 * }</pre>
 */

/**
 * Designed to be overridden, such as:
 *
 * <pre>
 * class Foo {
 *   {@literal @}Override public String toString() {return "";}
 * }
 * </pre>
 */

Suppression

Suppress by applying @SuppressWarnings("InvalidBlockTag") to the element being documented.