Kicker
Kicker , noun
1.
One who, or that which, kicks.
2.
A fact, condition, or circumstance, sometimes concealed or not obvious, which reduces or eliminates the benefit of an apparently advantageous situation; a joker{5}; as, under the Soviet system, bread was good and cheap, but the kicker was that you waited in line for hours to get any, if it was available. [informal]
3.
An unforeseen added expense or additional cost; as, the printer was cheap, but the special paper it needed was an expensive kicker.