I'm really not certain what the trigger circumstances are, but..
I was running about Nanawa mines with Forra doing leves, when the game took to flagging up the 'If you change Discipline during a leve you will suffer an experience penalty' warning.. although I admit I don't know the exact wording, but I'm sure you know the one - the message that should come up in the event I, well, change discipline during a leve.
One way or another however, well.. I hadn't changed class. It seemed to be occurring most when Forra drew his weapon, although I'm fairly certain that it also happened on one occasion when he used Doomspike.
As far as I can tell, I didn't actually suffer any penalties, but still, the error came up erroneously several times across four leves - and at no point did I change Forra's discipline from Lancer.
This was not happening yesterday at Halatali.. and as far as I can tell, the only difference between then and now (aside from location) is Forra's now running about in the Vintage Haubergeon, Cobalt Mitt Gauntlets, and Cobalt-Plated Jackboots I had him buy last night.