Heck, in many ways, double entry bookkeeping on the accrual basis would be less likely to notice it.

(I don't know whether the software used makes this easy?)

Btw, you could also fit this into the conceptual framework of accrual accounting, if you modelled counterparty risk.

From what I remember, in general you have to recognize outlays with certainty, but you have more leeway in how you treat inflows.

In general, you can also keep whatever you want in your books and run them however you like; you just have a legal obligation to also keep accounts that are in line with established standards.

You see this distinction in GAAP vs non-GAAP numbers in the US, where GAAP stands for Generally Accepted Accounting Principles.

In banking there's an obligation to model counterparty risk; but I have to assume that banks keep multiple copies of their balance sheets and accounts around:

One version for eg tax purposes, and another version for things like determining capital requirements and risk exposure; if different rules apply in the different domains.

It’d take both an open invoice and running an A/R aging to catch this one, which isn’t implicit in double-entry accounting.

Is it just me thinking this is actually crazy that they didn't notice even through a quarter closing? One could argue they're a small business but then how does the business not notice multiple months of AR being delayed.