To ESOP Guy: Nope, not a math error on my part. The total would appear automatically correctly, but then would change after I reopened the file. For example, I would input 3,333 into 16a(1) and 1,111 into 16a(2). 16a(3) initially would display 4,444, but after logging out and back in, 16a(3) would change to 33,331,111 (essentially a combination, rather than a sum, of the two values). When I call pay.gov they tell me they've never heard of this issue...
Such fun.