With the latest version of Jetpack (with the After The Deadline module enabled), and Advanced Custom Fields installed, there is a problem when trying to spellcheck.
Ensure the Visual Editor is enabled and create a new post with a misspelt word. Click the spellcheck button, and it will correctly detect the error. However, when you click the misspelt word (in Firefox at least), nothing happens, and an error is written to the console:
TypeError: this.suggestions is undefined – atd.core.js:266
It works fine if the Text editor is enabled; it also works fine if you switch to the Visual Editor after initially loading the page with the Text editor enabled. And it works in both cases if ACF is disabled.