We've had a problem the past few days that we've traced back to U-Boot. We're generating images using Yocto with Mender's update routine. The issue is the first time a clean image is booted on a Raspberry Pi, the mac address gets permanently saved to the ethaddr env variable. It's a combination of this line:
https://github.com/u-boot/u-boot/blob/b4ee6daad7a2604ca9466b2ba48de86cc27d38...
and what I'm pretty sure is Mender doing a "saveenv" on first boot. If you move the SD card to a different Raspberry Pi, it will boot up with the mac address of the first Raspberry Pi.
I'm not sure what the best long-term solution will be but I can make a patch for my purposes to just always set ethaddr to usbethaddr.
So I guess i'm emailing this in to report an issue and maybe help anyone else who runs into this issue.