Hmmm…I did everything except rebuild preferences (in my experience that never fixes my particular issues and I spend my day restoring custom settings), though I always repair OS X permissions and that almost always clears out the cobwebs. In this case, nothing helped, and the file was still acting up. I decided to create a new doc (something I should have done first) and typed the offending word many times. Ignore All did its job perfectly. Oddly, when I went back to the first file, it then got the memo that it was supposed to ignore the word and never even flagged it. My only thought is that some weird gremlin got introduced with the text I copied and pasted into InDesign (I write in Simplenote).