|This one is from my mother, who is fixing a wadder I made/messed up over the summer. She sent me an email last month, and mentioned it over the phone again last night: fabric marker ink can set when ironed ... and one of my markers definitely has that kind of ink. In fact, Mom says my blue fabric marker's ink turns brownish when it is heat-set, so I have started to spray a bit of water on the marks once they are no longer necessary if the mark will be seen on the garment - outside or in, just in case I have to send something to Mom to fix again.
ETA: Yet another fabric marker related DUH moment ... check even your air dry markers to see if they heat set ... and it's much better to check them on scraps first! I have just discovered my air dry marker heat-sets to a dark brown ... thankfully on a scrap. And hitting it with a can of compressed air does NOT make it disappear faster.