Please know that I am NOT an expert about this but from my research into LED bulbs, my understanding is this . . .

LED bulbs consume much less power than regular incandescent bulbs. The Crossfires (as well as most modern European vehicles) have a circuit that is designed to trigger a warning light alerting the driver when a light bulb is burnt out. As LED bulbs consume much less power, they will trigger that bulb out warning light. To prevent this, an inline resistor needs to be installed in the bulb circuit to "trick" the system and prevent the bulb out indicator. An ERROR FREE (or Can-Bus) bulb already has that resistor built into the bulb so there is no need to install a separate resistor into the bulb circuit. Buying ERROR FEE or Can-Bus bulbs is the easiest way to change over to LED lighting. I hope his explanation is understandable.