Virtual Keyboards
This article about virtual keyboards on iPhone and Android is a good case-study of how seemingly small changes can make or break a design.
It also contains this brilliant idea:
[David Pogue:] “Although you don’t see it with your eyes, the sizes of the keys on the iPhone keyboard are changing all the time. That is, the software enlarges the “landing area” of certain keys, based on probability.”
For example, if you type the letter «N», the next letter is unlikely to be «H», but quite likely to be «G». Accordingly, the iPhone will increase or decrease the target areas of these keys.