In Silence of the Library, River says to the 10th Doctor that he's younger than she has ever seen him, but we know that she met him in the Audio Adventures.
DOCTOR: For what?
RIVER: The usual. For coming when I call.
DOCTOR: Oh, that was you?
RIVER: You're doing a very good job, acting like you don't know me. I'm assuming there's a reason.
DOCTOR: A fairly good one, actually.
RIVER: Okay, shall we do diaries, then? Where are we this time? Er, going by your face, I'd say it's early days for you, yeah? So, er, crash of the Byzantium. Have we done that yet? Obviously ringing no bells. Right. Oh, picnic at Asgard. Have we done Asgard yet? Obviously not. Blimey, very early days, then. Whoo, life with a time traveller. Never knew it could be such hard work. Look at you. Oh, you're young.
DOCTOR: I'm really not, you know.
RIVER: No, but you are. Your eyes. You're younger than I've ever seen you.
DOCTOR: You've seen me before, then?
RIVER: Doctor, please tell me you know who I am.
DOCTOR: Who are you?
We know that she met the 8th Doctor in the audio drama The Diary of River Song, that were created later (in real time). So, if the Audio adventures were before Silence in the Library, she shouldn't have said that she has never seen him this you.
When she says that he's younger than she has ever seen him, is that a continuity error?
Or is there some other explanation for this seemingly mistake?