Note: this is available only during editing of text.

A temporary dictionary is built during spell check either when you skip a word by clicking Ignore All, or when you select a word, and click Correct All. Such words are automatically inserted in the temporary exception dictionary, and you are not prompted any more for then neither during the current spelling check,  nor in any spelling check you will perform later during the same work session.

To re-enable the prompt for such words, on the Edit menu, point to Spelling and then click Clear dictionary. This removes all words stored by Ignore All or Correct All and re-enables prompting on them.

Note: it is not possible to clear separately words added by Ignore All and words added by Correct All.