What should I call a balance sheet that doesn't balance?

It seems silly to have to say this, but you should call it what it is: an unbalanced balance sheet. And, clearly, given the definition of balance sheet, there can only be an unbalanced balance sheet if there is an error of some kind in the balance sheet: an error in the total assets of the company, the company's liabilities, the owners' equity, and whatever other pluses and minuses come into play. As @Greybeard has indicated in his comment, you could avoid the existence of an imbalance by including in the balance sheet an unaccountable deficit (or surplus) line. That would result in a balanced balance sheet in all cases, i.e., a balance sheet that actually balances. At the same time, it would indicate the existence of a problem (if the unaccountable deficit or surplus were significant) while shedding no light on the origin of the problem.

To be a hair more serious, one might say that the balance sheet has a "discrepancy":

a difference between two things that should be the same

You can use the expression “unbalanced balance sheet.”

M-W defines unbalanced as:

c : not adjusted so as to make credits equal to debits - an unbalanced account.

The are numerous usage examples of unbalanced used referring to a balance sheet such as:

How to Review an Unbalanced Balance Sheet. (smallbusiness.chron.com

5 Reasons Why Your Balance Sheet Is Unbalanced. (atsaccountinginc.com)

Unbalanced Balance Sheet & Trial Balance. (community.myob.com)