Of the top of my head...
People trying to mis-characterize leisure/personal travel as business expenses. Claims of S-corp business related deductions and credits are generally cited high audit trigger risks. I assume that is because the IRS at least believes they are widely abused.
Just as a general top line way to flag people who have life styles that don't seem align well to their reported incomes...and by extension are likely not reporting things they are required to do. The US tax codes is very well weird, we should never forget. You are for example required to report income from illegal activities, but the 5th amendment protects you from having to disclose what those activities are. So for an example that might fit here:
let's say fraudulently arranged some business travel for yourself to meet a client that does not exist, because you want to take a free trip to Monnaco on the company dime. That trip is income. As far as the IRS is concerned you need to report those 10k business class tickets and those 4k hotel fees you received. They may be curious if that was really a business and just how someone with AGI of 68,000 with three dependents managed to bank roll such an extravagant trip if it wasn't.