Occasionally, pickups spawned from killed monsters (in particular, the spheres created from the Crusader's Holy Strength ability) will become invisible if the game is saved directly next to them and reloaded. The items are still there, however, and can be collected as normal if you touch them.
I was able to reproduce this easily by killing an enemy until a sphere appeared, then saving and immediately loading the game just saved. I have verified that it affects both OpenGL and Software modes.
It appears to happen with other items dropped from killed monsters. Items placed on the map initially, or spawned from destroyed containers, appear to be unaffected by the glitch.
Currently using Version 1.5.0 rc-1. I am unaware whether this is present in earlier versions, or in the original engine.