I have seen this on rare occasions. Typically, I can never repeat the situation however.

Maybe try doing a rebuild key files/sort files?

We experienced the same situation back in November. The header memo line from paperless stated sending correct invoice attachment,yet totally different invoice attached.
5 different customers were impacted by receiving the wrong attachment.
A rebuild of the files did seem to remedy the situation.

I hate rare occasions ;)
I will run a rebuild this weekend, thanks.
PS we also continually have amounts under PaymentsToday in AR_OpenInvoice which I fix almost daily.

Rare occasions are definitely a b**ch.

Payment today amounts in AR_OpenInvoice are typically caused by starting a cash receipts entry batch and then deleting it. Shouldn't happen and in my experience it's one of those things that seems to happen randomly. Been around for years but has never been pinned down. Hurray for ARWAR4 to fix it.

Oi what a day!
We sent out 1400 emails last night, we've received 100s of calls/emails about incorrect emails. Customers not happy blah blah blah

I found the problem: Friday 13th!!!!!

Just remember not to re-send from the paperless viewer or it will send the same wrong one.
You will have to send electronically as if it were a new copy.

We've been manually emailing the correct invoices - don't trust Sage at this point.
The weirdest thing is if we look in the viewer then it is the correct invoice PDF for the correct customer. Somehow the email subject line and PDF got mixed up, and some customers received 2 different emails with different invoices.

do you recall any error notices during that day? We had a message Of "Unable to verify SMTP connection" the day we ran into this issue where the E-mail engine could not be started. Eventually message said "No PDFs can be E-mailed" when we fully locked up in electronic invoicing.

No errors reported by the user, but I'm starting to wonder.

Makes you wonder if a temporary file got started and not removed when the lock up occurred. Then when it restarted and succeeded there might have been remnants of the original run mixed in with the new run?

Seems like I remember that the one time I remember having this problem that there was some sort of hiccup in printing invoices but I'm not entirely sure what it was. I want to say that it had something to do with starting an invoice run, realizing that I selected something incorrectly and then restarted/reprinted the invoices.