WoW 8.3, 9.0 - Skillet (any 4.11+)

The CastSpellByName() function is reported as a protected function, but for crafting windows, pets and mounts it is supposed to be unprotected. For whatever reason the first time you load in a character those are still being registered as protected.

If you log out of that character and immediately log back in (do NOT close WoW.exe), the error does not occur, even if you do this before opening any tradeskill windows.

+Note that even making an in-game /script macro gets blocked as well (also supposed to be allowed; "resolved" the same way).