Here's another FSA to consider:

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It's fairly clear what it will accept: strings like "ab", "abab", "abababababab", and, of course {math}\epsilon{math end}. But there are some missing transitions: if you are in state 1 and get a "b" there's nowhere to go. If an input cannot be accepted, it will be rejected, as in this case. We could have put in a trap state to make this clear:

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But things can get out of hand. What if there are more letters in the alphabet? We'd need something like this: