It's one of those crisis-level-of-illiquidity things that if you heard about it in normal times it would make you shake your head in disbelief. During a full-blown meltdown maybe it's not standard stuff, but given the chaotic conditions, it doesn't seem so preposterous, either. Negative convexity is an otherwise benign phenomenon in fixed income that when combined with a lack of liquidity can rip your throat out in a heartbeat.
The reason it kills is how it can act like proverbial quicksand; you don't realize you are in big trouble until it's too late, and