52 Tiger:

Mac OS X Lion features automatic spell correction out of the box. The way it works is similar to autocorrect in iOS. As you type, Lion notices a misspelled word and offers a correction. To accept it, just keep typing.It sounds great, but in my experience it causes more errors that it prevents.

Dave is right, and I’ve seen enough people complain about this very feature to merit noting this helpful hint here. Visit 52 Tiger for details on how to shut it off.