Quote:
You gain +3 hit points. For every Hit Die you possess beyond 3, you gain an additional +1 hit point. If you have more than 3 Hit Dice, you gain +1 hit points whenever you gain a Hit Die (such as when you gain a level).
By a strictly textual reading the character taking this feat gets one less hp if they take it at 1-3 level.
If taken at 4th level you would get +3 hp, +1 for being one above 3rd, and then +1 for every level after that (for a total of +20 hp at level 20).
If taken at 3rd level you would get +3 hp, but when you level the last sentence does not trigger because you do not have "more than 3 hit dice" yet, and so you will still have only +3 hp at 4th level.
I think everyone agrees that's not how it was intended, though, so I don't see it being a large enough problem to require an official clarification.