5
A prize from a promotion you never entered
The oldest one, and still the most common.
You cannot win a draw you were not in. It sounds too obvious to state, and it is the single most frequently ignored fact in this entire category.
The counter-argument is always ready: you were entered automatically, or a friend entered you, or it was a partner promotion of something you did sign up for. Occasionally one of those is even true, which is why the scam works.
The resolution is to go and look. Legitimate promotions keep records of your entries and will show them to you when you log in to the account you entered with. If there is no account and no record, there was no entry.