I found a bug:

I have 3 addresses:

A = 0.1
B = 0.02111658
C = 0.48

I put these keys in various wallets:

W_1 = A+B = is displayed as 0.61111658 (wrong)
W_2 = A+B+C = is displayed as 1.09111658 (wrong)
W_3 = B+C = is displayed as 0.50111658 (correct)
W_4 = A = is displayed as 0.1 (correct)

C is a change-return-address from B. So maybe if you include B alone, it also counts C to it automatically? And if you add B+C together it counts C double. Then there are still discrepancies. Maybe because I ran synchronization multiple times from different "replay date"s? The transactions for W_1 and W_2 don't even add up to the sum that is displayed in the wallet. It's pretty chaotic.