my friend's parents, first stay here for 6 month, went back to China before I-94 expire, stay in China for 1 month, came here again since B2 visa still valid, but
入境時的 I-94 上蓋有“No AOS/COS/EOS”的戳兒, and give them only 1 month.
guess too many people use this trick and they found it out.