This could be a bug affecting the whole StackExchange network, but it has happened on StackOverflow (this is the reason I post this here)
I have posted earlier this question on Meta.SO :
How can an answer posted by a 3.5k rep user get used as an Audit in the "First-Post" review queue?
Short resume :
In the "First-Post" review queue, there was an obvious audit where there was a "First-Post" from a 3.5k rep user. It was obviously not a first post, nor a new user.
Screenshot taken before the audit was completed
Some are saying that the Usercard should of been "anonymized". This means that this is a bug (as it still showed the real Usercard).
(Not a duplicate to First Post suggests an answer from a "new user" who is 3+ years old with 1800 rep as the feature (anonymizing) wasn't up yet, now it is)