Most Common IRS Letters (Don't ignore them!)

The top three IRS letters that we see in our practice are the CP-2000 (the IRS computers think you forgot something), and two letters around balances due and payment posting issues. Have you received another letter that you think we should chat about?
Also, credit should go to taxaudit.com for the sample letters. I have no connection to them and thus can't endorse them, but I appreciate the samples.