Running status
MIDI's compression trick: leave out the status byte when it matches the previous message, and let the receiver assume it still applies.
MIDI 1.0, from 1983, runs at 31,250 baud. A Note On is three bytes — status, note number, velocity — and takes just under a millisecond to send. A ten-note chord therefore smears across ten milliseconds, which is audible. Running status lets the sender drop the status byte for as long as it stays the same, shaving a third off the chord.
That is why many keyboards send Note Off as a Note On with velocity zero: the entire stream of notes can then share one status. Real-time messages such as MIDI clock's F8 may slip in between without breaking it, while SysEx and other system messages cancel it. Parsers that forget either rule are a time-honoured source of stuck notes.