WHY is my interest from Savings Bonds in my Massachusetts state return? I thought that interest was exempt from state tax?

on what line of the 1099-INT was the amount entered? should be line 3 to be exempt. not all "US" interest is exempt from state taxes, especially certain interest paid by certain US government agencies

You are correct,  US savings bonds are tax-free on all state returns.  I suggest you return to the input screen for the bonds and double-check them. 

• Be sure to enter savings bond interest in box #3 Saving Bonds and treasury interest.
• If this does not resolve the issue go back through the Massachusetts state interview. Check in the Adjustments section.