Thankfully it was a missing digit, which should cause a bounce back.
Not sure what happens if one of the digits were fat-fingered and repeated on the confirm (copy and pasting the error).
I think some banks have a checksum in their account numbers also, so common errors such as simple transpositions or one-digit-off would not be a valid account number.
If not, though, it is possible that the fat-fingered account number is someone else's valid account. In that case, my understanding is that they get the refund, the IRS can't get it back, and the fat-fingering taxpayer is out their refund.
At my AARP Tax Aide sites, we enter the routing and account numbers twice into our software, and both copies have to match. (I don't know if we would be permitted to copy/paste, although I never do because doing so defeats the purpose of the double entry.) Then a second volunteer checks both numbers. Then the taxpayer checks both numbers and initials that it is correct.
My understanding is that there was a rather large refund for a taxpayer who went to an AARP site years ago, the volunteer fat-fingered the account, the reviewer didn't catch it, and the taxpayer was not asked to double check. The refund went into someone else's account and the IRS couldn't fix it and didn't feel obliged to as they put the money into the account number that was provided on the return. I think the AARP Foundation may have reimbursed the taxpayer but I am not sure of that. Perhaps @cathy63 knows the story more accurately.
Anyway, that is what was explained to me as to why we go through the process we do.