What do you need to do if you forgot to turn in interest earned on savings?

It depends on the amount of interest earned. If it's only a few dollars (as is the case with many savings accounts, which don't typically earn high interest rates), the IRS won't care. In fact, the bank isn't even required to turn in a 1099 form to you.

In cases of larger amounts (i.e., in the hundreds of dollars), you may have to file an amended return, which you should try and do before tax time.

It's best to contact your financial institution to find out, as well as call the IRS (you can ask the question anonymously so you don't get targeted). Overall, it shouldn't be a big deal if you aren't talking about a large amount of money.