Here and Now

Here and Now is a perk in Fallout 2, Fallout 3, Fallout: New Vegas, and Fallout Tactics.

Effects
This perk "advances the character to the next experience level, if available, with all advantages that entails" - Except one advantage. The level you just gained has no perk, as Here and Now effectively disappears after giving you another level. In simpler terms, you level up again at the cost of a perk.