This is something I spotted yesterday. It takes a little explaining, so bear with me.
Editing text on the iPad or iPhone is difficult. Well, it can be painful. But if I have to edit then tapping and holding a sentence or paragraph when the keyboard is visible will cause the magnifying glass to appear, showing the cursor position.
What I never realised is that while tapping and holding, and viewing the magnifying glass, you can still type to insert characters, or tap the delete key to remove them. Obviously you’ll need to use your other hand to do so. Inserting characters in this way is pretty slow on both my iPhone 5 and iPad 3rd generation – it takes around three seconds for the letters to appear or disappear – but it’s possible.
I’m not sure if this is an intentional feature. The slowness makes me think not, but it’d be awesome if Apple’s engineers made it so it didn’t take so long to register keystrokes.
UPDATE: This was written in the era of iOS 7 and doesn’t appear to work reliably on iOS 8. In fact, in my iPhone 6 Plus, only half the keys work when magnifying mode is activated. Weird.