The bug

/replaceitem wrongly reports "Block at pos(x,y,z) is not a container" when used on a barrel, and no item is placed.

How to reproduce

Run the following commands:

/setblock ~ ~2 ~ barrel
/replaceitem block ~ ~2 ~ slot.container 0 apple

→  Error message, apple is not placed in barrel.

Try again with a dropper or something and it works fine.