What happened

Wrong example for mergeDeep in docs. In live editor shown merge method which doesn't work as expected. If you run an example, output will be object with shallow merge and miscompare with expected output written in comments;
merge(original, { x: { z: 456 }}) // { x: { y: 123, z: 456 }}
https://facebook.github.io/immutable-js/docs/#/mergeDeep

How to reproduce

Just run example in editor from this page https://facebook.github.io/immutable-js/docs/#/mergeDeep
And see an output.