My friend Sarah had a similar thing happen with a knock-off curling wand. It actually left a small burn mark on her client's neck. She only uses professional tools for events now.
Used to think all flat irons were basically the same until I borrowed a cheap one from a friend and saw how uneven the heat was. @angelapalmer is right, that kind of heat damage is no joke when you're working with someone else's hair. Definitely sticking with trusted brands from now on.