Mark Weinrib asks about a typical typo:
I am trying to find out how to autocorrect double-letter capitalizations. For example, if I spell office “OFfice” I want to automatically make the F into a lowercase f. Microsoft Office has a setting for this. I can not find one in OS X, so when I am in Mail or` in TextEdit it doesn’t autocorrect.
OS X does offer this setting system-wide as part of automatic spelling correction, and it works in all of Apple’s programs and is available in other apps that take advantage of it. This autocorrect will drop in what OS X thinks is the “correct” replacement as you type—it may sometimes be the wrong one if you’re using a specialized term or a special spelling. Generally, it’s the right choice.

OS X can automatically correct words, and it alerts you after a moment to potential corrections, too.
This includes accidental miscapitalization. Type “OFfice” and the capital “O” is preserved, as autocorrect won’t set a capitalized word to lowercase; but the first “F” will transform to its downshifted equivalent after you press the spacebar. If you type any other punctuation after a the word, while the option will put a wavy red underline beneath the word, it won’t correct it—unless you wait for a brief moment for a pop-up to appear with the corrected word shown in blue. You can type any other character or click the blue word to accept the change, or click the X next to the word to reject it.
In System Preferences, you can enable autocorrection in the Keyboard pane under Text. Check the Correct Spelling Automatically box. In individual apps, like TextEdit, Pages, or Mail, use the Edit > Spelling & Grammar sub-menu to make sure both Check Spelling While Typing and Correct Spelling Automatically have checkmarks. Many third-party apps have the same settings, picked up from the system.

Keyboard preferences let you set automatic spelling correction system-wide, but you can modify this in each app that supports it.
You can also get this feature everywhere you type through TextExpander. The program monitors all your keystrokes in OS X, and among a variety of features can fix double-caps in a couple of different ways.

TextExpander offers specific options for correcting the common error of typing two capital letters in a row.
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