I used to think that what happened there was:

1. Liz got the two confused
2. Fred corrected her
3. Liz forgot about the correction
4. Next time she saw Fred, she made the exact same mistake again and called him Rick

But now I am realizing that what actually happened was probably:

1. Liz got the two confused
2. Fred corrected her
3. Liz took mental note of the correction so she does not repeat the mistake
4. Next time she saw Fred, she genuinely thought this was not the same guy she had the interaction with earlier. She instinctively wanted to call him Fred, but made the conscious decision to call him Rick