When a barrel is crushed, it leaves behind a pool of blood and crushed bones. Normally, monsters leave behind a pool of blood when crushed. Barrels leave a pool of blood too because the code that draws the pool of blood does not check to make sure a crushed object is not a barrel. Barrels never appear under a crusher in any official IWAD, but they may appear in PWADS. This bug is fixed in ZDoom, but it is still present in the Boom family of source ports.