It has been said sometimes that a Bitcoin transaction can be cancelled or reversed using for example double-spend.
Is it still possible? If Yes, how? If No, Why?
Especially when you sell services and goods and accept Bitcoin payment, you are worried about this.
So it should be a good idea to discuss this topic.
That is not a cancel, and is not a reverse, that is just what you yourself said: A double spend. Only one may enter the blockchain, and that obviously is the one paying the higher fee, even when broadcasted later. Your window of opportunity could be a a small as the time remaining to find the next block, so its not even guaranteed you would succeed.
Unfortunately the likely people making the mistake probably didn't even bother lowering the tx fee shrinking their chance.
Moral of the story: triple check where you send your coins to.
You CANNOT reverse or cancel a tx.