As Bruce Alderman's answeranswer ably points out, we can never know the exact words that Jesus spoke as there were no recording devices at the time. Therefore, we must rely on the people who heard Jesus' words to remember them until they could be recorded in writing. For a critical 30 or 40 years, the teaching must survive in the form of oral traditions. There is very little evidence for what, if any, changes might have occurred to the teachings in this period.
Another answeranswer by jrdioko points to the idea that the phrase is midrashic or that it should be understood as a general marker of a time period. In these cases, Abiathar shouldn't be considered a mistake so much as an interpretation problem to be solved. Using the name of one priest rather than another might have been intended to carry a deeper significance. Unfortunately, that Luke and Matthew both drop the phrase indicates that, at the time they wrote, the reason for the substitution had already been lost. If it had been retained, the authors would have had reason to preserve the text or even elaborate on it.