Courtesy of @MikeyBurkman, reported in #11597 (comment)

/**
 * @typedef {Object} Opts
 * @property {string} x
 * @property {string=} y
 * 
 * @param {Opts} opts
 */
function foo(opts) {
  console.log(opts);
}

foo({x: 'abc'});

That call to foo() will be flagged because it's missing y in the opts. The type checker does not recognize any of the methods of specifying optional parameters listed on the wiki. I even tried doing TS-style using @property {string?} y and still no luck.