In one case, I got an audit letter, said it was a random audit, and everything checked out. A second time, I got a letter that I made an addition error (in their favor), and ended up getting another couple hundred bucks back.
An audit can also result in them asking for a paper trail to back up what you claimed. Keep in mind that many of the common items (interest earned, mortgage interest paid, etc) are all filed with the IRS by third parties, via a 1099 or 1098 form. But for some other items that you can claim deductions on, such as business expenses, you will need to provide them a copy of the receipts if they ask. If you can't back it up, they adjust what you owe, and may apply interest / penalties, which you will have to pay. If not, they can do things like garnish wages, freeze bank accounts, etc. But that will come after a court judgement typically.
Keep in mind that some of the items they will go after: If you turn in expenses at work, and they reimburse you for that, the IRS may still require you to retain the receipts to show that it is a valid business expense that you got paid for -- otherwise they can consider that income and tax you on it.
