I have a particle system acting as a star field in the back of my player. it looks really nice, but the stars have to have a long lifetime (20+ seconds).
This long lifetime means that if the player starts to move quickly in any direction the particle system maxes out, even with a huge max particles number, and they fly out into deadspace with no stars behind them.
I could probably detect if the particle has left the screen / or an area a touch bigger than the screen and clean them up. But it would be much simpler if there was some way to set a particle system to overwrite the oldest particle when its reached its max.