Irons can accumulate debris over time by being used on small plastic buttons or blends of fabric with too much plastic. One simple way to fix this is by quickly ironing a dryer sheet.
Photo by Pete.
Real Simple's website offers this method:
Remove gunk from the soleplate of an iron. With the setting on low, rub the iron over the dryer sheet until the residue disappears, and you're left with a pristine press.
Give it a try next time you use an iron on something you probably shouldn't have.