MAVLink invalid prefix (42)

Hi, I have been working for some time with MAVLink, and is getting a quite good understanding of how it works. But one thing that keeps getting to me, is a invalid prefix. I know the prefix has to be 254, but I often get a lot of other prefix (even at USB connection, so no dirty links). Mostly on boot. Is this related to the boot, and something that happens mostly on boot? Or what does it mean.