Frequency offset (FO) is an inherent feature of wireless communications. It results from differences in the operating frequency of different radio oscillators. Failure to compensate for the FO may lead to a decoding failure, particularly in OFDM systems. IEEE 802.11a/g systems use a globally known preamble to deal with this issue. In this paper, we demonstrate how an adversary can exploit the structure and publicity of 802.11a's frame preamble to launch a low-power reactive jamming attack against the FO estimation mechanism. In this attack, the adversary will need to quickly detect a PHY frame and subsequently distort the FO estimation mechanism, irrespective of the channel conditions. By employing a fast frame detection technique, and optimizing the energy and structure of the jamming signal, we show the feasibility of such an attack. Furthermore, we propose some mitigation techniques and evaluate one of them through simulations and USRP testbed experimentation.