A strange time-sensitive bug in my application, running on Teensy++ 2.0 and Teensyduino 1.15, finally distilled down to this:
byte buf1[] = { 'a', 'b', 'c' };
byte buf2[] = { '>', '>', '-', '-', '0', '1', '0', '1' };
byte buf3[] = { 'x', 'y', 'z' };
Serial.write(buf1, 3);
//...