In 26:8 it says, "When Yitzhcak had been in Grar a long time..." then the story happens that Avimelech catches Yitzchak and Rivka acting like husband and wife rather than brother and sister...
Putting on an act to try to convince someone of a specific reality is not an easy feat. Much detail needs to be prepared and it would be very easy to slip and and mess up the whole act.
When the act is extended, as it was here, it becomes much more difficult. not only did they have to act as brother and sister for a few days without anyone catching them, but they had to do it for months or years. That is much more difficult. After a while of doing it successfully, you get complacent. You figure you have it down pat and might not pay attention to the small details anymore. That leads to your being outed.
That is what happened here. They were doing a fine job of making Avimelech think they were brother and sister. But once they got stuck there and had to keep up the act for an extended amount of time, that is when they faltered.