Sometimes spell checking is distracting for the user, e.g. when dealing with e-mail addresses, code, or (in my case) writing an application that allows users to learn words.
It should be possible to disable spell checking in input fields or textareas like in Firefox (see the URL), using the spellcheck attribute:
<input type="text" spellcheck="false">
*** This bug has been marked as a duplicate of 14552 ***