My local fire department, @Barrie_Fire, recently tweeted (then subsequently deleted) this:
Don’t even THINK about using a 2-prong plug in a 3-hole slot! Use only the required number of slots in an outlet or power strip.
Below was a picture of a burned-out grounded extension cord.
I'm hesitant to argue with anyone in the business of keeping our food, shelter, clothing and loved ones from combining with oxygen, but this seemed quite strange; I can't think of any possible way this could be a fire hazard.
The NEMA 5-15 wall receptacles in Canada are grounded by default, for reference.