Some commands like /particle and the /execute command itself aren't executed at the specified position (x, y and z parameters):
/execute @p ~ ~2 ~ /particle barrier ~ ~ ~ 0 0 0 0 for example shows the particle below the Player's feet, however they should be displayed above his head (at least by the description on the Minecraft Wiki: execute <player|entity> <x> <y> <z> <command>: Runs command as though it was executed by player|entity, and as though player|entity was located at x, y, z [...])
Other commands (e.g. /setblock and /fill) do work as intended.