Unstick the Spelling Checker
The Problem:
Every now and then, the spelling checker has trouble with a particular word and checks it repeatedly until I cancel the spelling check.
The Solution:
This normally happens only when you click the Change All button in the Spelling and Grammar dialog box, the queried word isn't in the main dictionary or a custom dictionary, and the document contains multiple instances of the word.
The easiest solution is to cancel the spelling check and use Replace (Edit 
Replace) to replace all the instances of the offending word. Alternatively, if the word is correct, add it to the main dictionary or a custom dictionary.