The backspace button could be an option when fat fingers press the wrong adjacent key on the small phone keypad. All too often my aim is not the best and I'll get the character next to the intended one. The latency problem also gets me sometimes and I'll type the same character twice because the first time it didn't enter but it was the latency problem. Of course, the backspace could be abused, so set a "3 strikes your out" limit and mark it wrong after three attempts.
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