Not sure if this is Doomsday specific or if it happens in vanilla Doom but the Imp's fireball (and maybe other projectiles) moves over slightly elevated parts of a map instead of colliding with it. Once the fireball passes over the elevated geometry it moves back down and continues on its original course. A map to observe this would be map01 in the room with imps. Stand behind that thing on the ground that triggers the rocket launcher secret and watch the imp fireballs bounce over it and then back down before continuing. It doesn't just happen here but anywhere where a part of the map is only slightly higher than the fireball. I've seen this in older versions of Doomsday and assumed it would eventually be fixed. Now i'm wondering this is a known bug or if it isn't a bug at all.
*Oops! Didn't mean to post this in Tech Support*