I loaded map 01 of Doom II on Ultra violence. I summoned a friendly cyberdemon at the beginning of the level, I went through the level as normal and spawned a few more friendly things along the way. Then I heard the cyberdemon shooting rockets at something, so I went back to the first room and I saw this:

He was shooting at the wall, and the rockets were floating in midair as if they had 0 speed. I walked into one and killed myself.
Then, It happened again under similar circumstances. I took the screenshot.
I can reproduce this, but I haven't the slightest clue what causes it.