Sometimes a user will get the confusing Joomla message :
Warning: JUser: :_load: Unable to load user with ID: 62
What does it mean?
Well, Joomla records which user wrote/edited each article in the article manager.
This error message appears when one such user no longer exists in the system, but is referenced as one who edited the article.
In other words, it is not really an error to worry about.