If several actors with a visible sprite are in the exact same spot their sprites flicker whenever any other sprites enter the player view. It basically seems as if ZDoom couldn't decide which of the stacked sprites to draw in the foreground and which in the background, switching between them whenever other sprites show up in the player's view.
I whipped up a little sample Doom II MAP01 wad for reproducing the bug. Just make sure you have rocket trails enabled, pick up and equip the rocket launcher right at the start, fire it and watch the stacked gib piles and rocket launchers flicker at an insane rate.