Note that as of 20w07a, a 3 tick repeater is needed, not 4 ticks as in the attached screenshot.

If you fill a dispenser, aiming upward, with flight-duration-3 fireworks and then push the firework entity horizontally with a slime block attached to a sticky piston (with a delay of one setting-3 repeater between the dispenser being triggered and the piston extending) it launches the firework diagonally with a ridiculous velocity.

Sometimes when a firework is launched in such a way it causes the game to experience crippling framerate lag or even temporarily freeze up. Hooking the device up to a clock can have terrible effects on a world until the clock is disengaged. In 1.8-pre2, launching a single firework in this way crashed a singleplayer world.

Flight duration 1 and 2 do not seem to cause this problem. I launched a few of each and there were no adverse affects. It seems that it might be from the firework reaching unloaded chunks without managing to explode. I was using 16 chunk render distance. Once I changed it to 32 chunks I could launch flight duration 3 fireworks with no adverse effects.

Edit: After switching back to 16 chunks the issue vanished. It seems it might be caused by fireworks reaching ungenerated chunks without exploding. Or possibly not. The lock-up happened again after launching a lot more.