If you spawn in a particle using the :Emit() method while the particle is offscreen, wait a long period of time, and then move the camera so the location where the particle should have spawned a while ago is on screen, the particle will spawn and play through its sequence even though time-wise it was supposed to have spawned and despawned ages ago.
This issue happens with 100% consistency. An easy setup to demonstrate this bug would be to place a part on top of a blank baseplate, insert a particleemitter into that part that is disabled, make sure the part is complete out of view of the player’s camera when the player spawns into the place, write a script to call the Emit function on the particleemitter once upon loading, and then after several seconds, turn the camera around to point towards the part to observe the particle be emitted at that exact moment.
Here is a video demonstrating how this bug occurs. You can see that in the first playtest, the particles spawn as soon as the “emitted” text is printed, and disappear as soon as the “emit ended” text is printed. But, in the second and third test, since the particles are offscreen, they will appear way after the “emit ended” text.
Baseplate.rbxl (18.4 KB)
Here is the test place I have created to demonstrate this issue in the video.
I know this bug has been reported once prior on October of 2018, but that post lacked a video or test place to help reproduce the issue, and it seems like the bug report was missed under the pile of hundreds of other bug reports, so I think there is a valid excuse for this repost.