Add a function with a command such as setblock ~ ~2 ~ stone or say @s.
In the world place a command block and run the function as a player (or other entity):
/execute @p ~ ~ ~ function foo:bar
The expected behaviour is that a stone block is placed above the nearest player, however, it is placed at the command block instead. As well, using @s in the function does nothing, as the command block is the sender.
Affects similar commands in other contexts as well.