We study the performance impact of the Message in Mes-sage (MIM) mechanism in modern 802.11 networks. The MIM mechanism refers to the capability of receiver to aban-don an ongoing reception of an 802.11 MAC frame and shift to decode another frame with a higher signal strength. MIM is a common feature in modern 802.11 adapters and it has been shown to improve spatial concurrency. However, our measurement study in a campus WLAN shows that under certain conditions, MIM could cause a throughput degra-dation of more than 30 % when enabled, instead of improv-ing it as expected. With comprehensive experiments using commercial 802.11n adapters, we characterize the impact of MIM for a range of parameters and for different scenarios. We show that a simple adaptive MIM scheme can poten-tially achieve throughput that is close to the optimal. Our method is practical because it can be easily implemented in existing commodity 802.11 adapters.