Feedback aggregation is an approach for repeat request protocols in which positive and negative feedback from multiple wireless stations is aggregated within a single time slot, during which negative acknowledgments (NACKs) from non-leader receivers may cancel positive acknowledgments (ACK) sent by the leader. This solves the problem of increased overhead with increasing number of receivers, and enables high rate audiovisual mediacast for large wireless multicast receiver groups. In this paper, we investigate feedback cancelation (jamming) failure rates, which in turn determine the performance of error correction protocols using feedback aggregation. We do so by simulation and measurement using a variable number of NACKs in i.i.d. fading scenarios. Our results apply to OFDM based wireless multicast and we show that multicast error performance and throughput can be tuned with ACK power control using consumer IEEE 802.11 hardware.