@jason192 

RE: It turns out the problem was that my bookkeeper had created "credit memos," and the numbers assigned to those credit memos were also treated as *invoice numbers* within QBO.

Another basically idiotic change by QB Online. What are they thinking?

Invoices and Credit Memos are not the same thing, and IMO shouldn't even share the same numbering sequence.

And, in any case, who cares one whit if you want to use the same numbers more than once? Good grief.