This bug allows you to get a specialist level at level 1, without being level 21. It simply lets you get your first specialist level from the start, and you can't get any more specialist levels until you reach 21.
Bug Recreation:
Go through normal character creation until the point where you pick feats. Pick Skill Focus, the second feat doesn't matter. Once you're done with that, go back to the class selection. You will now notice that you can pick Specialist as your starting class. You can do so, and go through the rest of character creation as normal, thus creating your very own level 1 specialist character.
Expected result: Not applicable.
Actual Result: A purely level one specialist character. (See screenshots)
Misc Notes:
The only requirements for Specialist are being level 21 and having any skill focus, so during character creation when you have no levels, you can meet the skill focus requirement, thus letting you go back to character creation pick specialist. This doesn't work with any other prestige class, oddly.
Screenshots:
