Thank you for responding. I may be looking for it in the wrong place, but I haven't been able to locate any unapplied payments for $166 that could be assigned to the bad debt account. I can enter a payment to the customer's account dated 12/31/2016 and that eliminates the 2017 opening balance. However, the customer didn't actually make that payment, so this seems like sweeping the problem under the rug.
I went back to the customer's 2016 invoice and reentered the $166 charge (it had changed to zero when I transferred it to bad debt, although it doesn't show up in the bad debt account now), with the idea that this would then create an amount due of $166, which I could transfer to the bad debt account. However, the account shows an available credit of $144 that shouldn't exist, so QB won't let me transfer the $166 to bad debt without first applying the available credit. Checking the other customer accounts, almost all of them show available credits that shouldn't exist. Any idea what might be causing this? Clearly, there's a lot I don't grasp about how QB operates (my previous experience was with GnuCash).