"/setblock ~ ~2 ~ dirt" places above your head, even when on a halfslab.
"/execute @p ~ ~ ~ /setblock ~ ~2 ~" also places above your head, except not when on a half slab. It places in your head.
This also can be seen if you:
/summon Fireball ~ ~2 ~ {direction:[]}
/execute @p ~ ~ ~ /summon Fireball ~ ~2 ~ {direction:[]}
Video of setblock for evidence / unmistakable ability to recreate:
http://youtu.be/ekbPS_XoCV4
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Confirmed for 1.8.2 pre-1. MC-76491 has better information, so I would suggest that become the main post.

Relinked to MC-67515
This isn't a dupe of MC-67515, although it is related.
This is not a duplicate of MC-67515. That report is about command blocks executing from their center, this report is about relative coordinates being treated differently depending on whether they are in an execute command or not.
Agreed, reopened.

Dupe of MC-105820
Indeed same cause.
Confirmed.