I've noticed this bug previously as well but couldn't get you a "live" one so you could have a look at it for yourselves.
Basically, let's assume there is a user by the user name "123" which is changed to "789".
All posts which 123 had liked still show as "123 liked this post", instead of changing to "789 liked this post". I noticed this once, and since I was about to like the same post as well, after liking it, 123 automatically changed to 789.
And this is probably related to the above. http://xenforo.com/community/members/brogan.521/?page=3#profile-post-4816
The user EntropiaPlanets has been renamed as SchmitzIT, yet it shows as the previous one.