It's not something I would encourage anyone to amend a return for, and if we're being honest it's also not something I'm sure the IRS even tracks. The IRS already gets that information via form 5498 filed by the custodian of the IRA which tells the IRS the amount contributed in the year and also what the value of the account was at the end of the year.
I've not encountered any issues with clients who hadn't previously reported roth contributions reporting tax free distributions on their return.
Whether you've kept track of the contributions or not, the key is to be able to support the tax free distribution with evidence that you in fact contributed the money.
Fair enough - I stand corrected. That's a good point about the 5498 going to the IRS. Maybe it's not as big of a deal as I've been led to believe. I haven't yet had a client in this situation, partly because I remind them to report the contributions, and partly because I'm usually able to come up with a better option than withdrawing their contributions. Unfortunately, in this case, there might not be a better option for you.