i noticed recently that units that are upgraded, say from an adept to a conjurer, can immediately use their free promotion. this seems incorrect to me, and means if you just leveled up, you can upgrade to get the free promotion which will then let you use a level you just earned(which you would otherwise only be able to use on the following turn)
but the real exploit, cast a spell with an adept, upgrade to a conjurer, and you will be able to cast again, since you can use the free promotion, this means you can cast a conjurer(level 2) spell.
granted its a pretty limited oppertunity exploit, really only adepts to conjurers/mages, and to a much rarer degree, conjurers to summoners, mages to archmages(with only 3 allowed, really rare, but the spells being more powerful), but if set up right, could be pretty nasty.