People seem to enjoy pointing these things out, so why not devote an entire thread to it?
Example 1:
CALI
In FFX you see a little girl on the Mi'ihen Highroad named Cali, then two years later she gets a little taller, gets a nice tan, and changes her wardrobe! FFX-2 Cali clones can be seen in several places in FFX.
1. In the stands during the blitzball tournament in Luca.
2. I think I saw her in Kilika.
3. In Remiem (that seems off, I must've gotten that wrong) Temple you see Cali again, as a fayth of one of the Magus Sisters!
4. I know there was some other fourth place! Someone help!
Example 2:
RED HAIRED GIRL
1. During the little party before the game starts in Zanarkand.
2. During one of Tidus's flashbacks of Zanarkand (though it's not so odd that she'd be seen in two different places in the same city...I mean she lives there....)
3. You see twin red-heads on either side of Rin during the Sphere Break tournament in FFX-2.