During the scene at the Mi'ihen Highroad Travel Agency, when Tidus steps outside, and is looking at the sunset, we see Auron and Wakka are outside, just in front of the Travel Agency.
During the conversation between Tidus and Yuna, this is where you need to keep an eye out for the first part of this slip up. As they are talking, you can see the front of the Travel Agency in the background. But, looking close enough, take notice that Auron and Wakka have disappeared, and in just about every shot that you can see of the Agency during the conversation, they are still missing.
So where did they go? We know the following facts:
1. Wakka couldn't have gone in the Travel Agency, because, since the Travel Agency was an Al Bhed shop, and Wakka doesn't like the Al Bhed, (so, this being the reason he was outside, it’s not likely that he went in there.)
2. Auron re-appears and interrupts the conversation. For him to know what was going on in the conversation, it isn't all too likely that he went into the Travel Agency either (Since he would have had to been there to catch most of the conversation.)
With the second fact in mind, another slip-up occurs. When Auron interrupts the conversation, he is standing in a different place than where he was before the first slip-up occurred. But during the conversation, there are some shots that you can see the exact same place where he re-appears. But, he's not there either!
(I'm pretty sure I saw this slip-up, in case you are wondering.)