- Fixed: Missiles with MF6_STEPMISSILE set would be allowed to cross lines they could step over,

but their Z position would not actually be moved up, so the subsequent call to P_ZMovement()
  would destroy it because it was in the floor.

SVN r3491 (trunk)

src/p_map.cpp

@@ -1788,6 +1788,15 @@ bool P_TryMove (AActor *thing, fixed_t x, fixed_t y,
 				{
 					goto pushline;
 				}
+				if (thing->flags & MF_MISSILE)
+				{
+					thing->z = tm.floorz;
+					// If moving down, cancel vertical component of the velocity
+					if (thing->velz < 0)
+					{
+						thing->velz = 0;
+					}
+				}
 			}
 		}