OK, SD - I'll bite and give you a tricky one.
Here in Vegas, many sportsbooks have mobile betting apps. You bet through an individual account, and books are able to report your win/loss statements to the IRS, etc, upon request. As a result, accurate record keeping is a must.
Well, in some cases, you might bet with someone on your account ("I go half, you go half?"); or even for someone else entirely on your account ("Oh, sure, I can bet that for you!"). Let's assume by the end of the year, you won 10k at a particular sportsbook, with 7k belonging to you, and 3k belonging to another person.
Should you report the entire 10k and pay taxes on it? Or should you only claim your personal share? Remember, the sportsbook has your account info and shows that you won 10k, and that is what is reported to the IRS.