My logfile has 1877 lines, 1609 are this kind:
Warning: creature_picks_up_trap_object: Cannot deliver crate to position of BOULDER trap index 415
I was playing the Kari-Mar level (Deeper Dungeons) when this log was generated. I remember that playing this level, sometimes my imps cannot left the trap box in the boulder place, so I loaded my savegame again and did it again. I cannot remember why this happened to me, possibly because the enemy was near and the imp stopped, but later, when the enemy was far, the imp didn't move the trap box to the boulder I think.