I get the same bug whenever I log in as a Shadow Priest:
FrameXML\PriestBar.lua:79: Usage: UnitPower("unit"[, type][, unmodified])
<in C code>
FrameXML\PriestBar.lua:79: in function "PriestBar_Update"
FrameXML\PriestBar.lua:63: in function "PriestBarFrame_CheckAndShow"
FrameXML\PlayerFrame.lua:734: in function "PlayerFrame_HideVehicleTexture"
FrameXML\PlayerFrame.lua:386: in function "PlayerFrame_ToPlayerArt"
FrameXML\PlayerFrame.lua:143: in function "OnEvent"
FrameXML\UnitFrame.lua:563: in function <FrameXML\UnitFrame.lua:561>
This error occurs in v3.8.6-1-g29e9dbd (alpha) without any other interfering addons (except BugSack). The error does not occur in v3.8.6 (release).
0
Aye, same happens to me.