Presumably this is happening with any item that gives you extra uses of feats/enhancements.
I noticed that each time I swap off either my Encrusted Ring or Knost's Belt I lose one rage. I should lose the extra rage granted by the set bonus, but never the rages from class levels and enhancements. However, each time swap the item off, I lose another rage. I can repeat this until I'm at 0 rages.
As reported in the Fighter forums, same think is happening with Verik's, regardless of if you have the set bonus because the necklace gives you 2 extra action boosts itself, not as part of a set bonus. The only difference is here you lose two action boosts until you hit 0 each time you swap it off.
I didn't see where it was mentioned that this was happening with the Frenzied Berserker set, so I thought I'd mention it here. I already submitted a bug report.
Is this happening for anybody else with different items/set bonuses? It looks like items that give extra uses of feats or enhancements are subtracting their bonuses whenever they are unequipped.