It is a "mistake" as soon as the work gets to be too much for one developer and the company brings in some more developers.
By this time, that one inexperienced developer gets promoted to "Architect" because he knows where all of the bodies are buried. Now one of two things happens. He brings in more inexperienced developers because he doesn't know what a good developer is and they keep adding on to the UserManager class and to organize it better, they add section breaks. The class grows.
If they are lucky, they get an experienced developer who tries to tell the "Architect" about proper coding techniques, management and the architect dismisses the critique as "if it ain't broke don't fix it"/"this is the way we've always done it". All of the good developers leave and you are left with only bad developers.
Then you end up with the "Dead Sea Effect"....
http://brucefwebster.com/2008/04/11/the-wetware-crisis-the-d...