Yikes - presumably not while you were in it? Have to say it's been blissful this week - just for a quick boost to temperature getting into bed. Don't leave it on for long though & hopefully have more modern precautions against bursting into flames.
A friend quite dramatically set fire to their flat with an electric blanket. Modern one, switched on when they got in of an evening, and in the time it took for them to make tea and get ready to turn in, it had set half the bedroom ablaze...
Had one set fire to the bed years ago and would never use another.
Hot water bottle between my feet is good enough for me.