Thanks for your reply.
I found out that the problem was that I entered the wrong code in Box 7 of an 1099-R form. With this correction Form 1040 line 1 is now exactly equal to the sum of W-2s. This was difficult to troubleshoot because TurboTax's supporting form link took me to the list of W-2s. 1099 that was included in this sum was not listed at all.
