My iron picked up the graphite from my pencil and I didn’t notice until this…….
pencil markings on the fabric.It won’t come off so I cleaned my iron. I don’t use anything fancy, just a plain brown paper bag.
I leave the iron plugged in and run it over and over on the bag checking once in awhile to see how clean it is getting.
I’ve been known to do this if there are stubborn spots. 
When its all clean
the plate will look spotless and the brown bag………..
well, not so much.
Always keep a pressing cloth at hand when using fusible web and make sure you lay it over the piece you are pressing. It will save you a lot of angst. No, it wasn’t something I did that day. I hope I’ve learned my lesson.