ap_start_lingering_close flushes the output buffers to the client. Therefore it should not be called from the listener thread unless we know that the output buffers are empty.
Fixed in trunk with r1294356 and in 2.4.x with r1364613. Released with 2.4.3. I guess even if it would apply to 2.2 we wouldn't port back to the experimental MPM there so closing now.