You receive a call from someone claiming to be from the Internal Revenue Service (IRS) or the Treasury Department saying that there is a warrant out for your arrest due to unpaid taxes. They tell you the debt can be cleared, and therefore the warrant voided if you pay them immediately. You are directed to go to a grocery store and purchase a gift card or "green dot" card and call them back with the numbers. This will clear your debt and save you from the embarrassment of being arrested.
This is a scam. The IRS will never call you like this for unpaid taxes. You will always receive an official letter in the mail. In addition, they do not issue warrants for your arrest like this.