Didn't see this anywhere and maybe it isn't that useful, but sure does a little bit of difference.
If you have [Presence of Mind] and [Arcane Potency] in your offspec and using your main spec maybe FFB or TTW for PvE, you use [Presence of Mind] to get [Arcane Potency] then if you change spec to your mainspec your [Presence of Mind] gets removed bu your [Arcane Potency] will still be there. That'll give 30% bonus crit for your next spell.
Something i found out by a mistake.
Dunno if this should be in the exploit section or guide section, so i just posted it here.
Edit: The title should be "+30% Crit First Spell Cast (Mage)"