[sfx]

In versions of PICO-8 prior to 0.2.2, and according to the manual:

SFX instruments are only retriggered when the pitch changes, or the previous note
has zero volume. This is useful for instruments that change more slowly over time.
For example: a bell that gradually fades out. To invert this behaviour, effect 3
(normally 'drop') can be used when triggering the note. All other effect values	have
their usual meaning when triggering SFX instruments.

However, in version 0.2.2, these SFX also retrigger when they have exceeded their total length.

The SFX up top demonstrates this: in 0.2.1b,

• SFX 03 plays a single 16-tick note three times, 128 ticks apart, and
• SFX 04 plays eight 16-tick notes at equal 32-tick intervals;

but in 0.2.2c,

• SFX 03 plays a 64-tick note, two 16-tick notes, and then two 16-tick notes, and
• SFX 04 plays eight 16-tick notes at a syncopated rhythm, alternating 48 and 16 ticks.

This will break some pre-0.2.2 compositions.

p.s. Slight edit correction: the manual is also incorrect in the opposite direction, because custom SFX instruments do not retrigger if the previous note has zero volume but uses the same pitch and instrument.

Just checked - this is still a thing in 0.2.4. As far as I can tell, this change has no musical use; any case where it would be helpful could be better handled by making the original custom SFX instrument looping.