When OS X Lion was first released, I had a heck of a time with its built-in spelling correction system. It's not that I don't need a spell checker; believe me, I do. But Lion's auto-correct feature makes changes so quickly you may not even realize that anything was changed. In addition to potentially embarrassing results, I don't want a spell checker to make changes without at least asking if I really meant to say what I said.
Screen shot courtesy of Coyote Moon, Inc.
OS X Mountain Lion has the same aggressive spell checker. If you upgrade from Lion, Mountain Lion respects your current spell checker settings. But if you perform a clean install, or upgrade from Snow Leopard, you may want to check the system-wide spell checker settings before you send your next email or business correspondence.
You can find complete instructions for taming the automatic spell checker in OS X Lion Automatic Spelling Correction.