[I'll apologize in advance if this comes out accusatory or otherwise offensively. I'm not trying to offend - I'm just trying to shed a different perspective on things.]

I don't have ... the coveted YNAB buffer established yet

A buffer would have helped the symptom, not the problem.

wouldnt you have a surplus from last months budget ...?

... so I used the money from last month already.

This is the problem.

If you are using YNAB (or Quicken or Money or Mint I assume), then you should be able to see from the uncleared balance and/or category balances how much money you actually have left, regardless of whether the "check cleared". If someone delays in cashing a check (or in this case, if autopay fails to go through), it should have no effect on your spending, because you should already be counting the money as gone, i.e. as if the check cleared and the money left your account the instant you sent it.

It sounds like you may be making spending decisions based on the (cleared/online) account balance instead of category balances. Though you had to spend over your budget to deal with the unexpected mishaps, a tool like YNAB et al should be helping make clear that you were spending with money you technically don't have any more (via a low balance or negative balance or category).

The real PSA is to not get fooled by the cleared balance.

YNAB and other budgeting apps take this a step further. Just like the checkbook register helps you keep track of the uncleared balance (i.e. all the money you've already promised other people in the form of yet-to-be-cashed checks/unpaid credit card balances), YNAB and budget categories also help you keep track of the money you've already promised to your future self for things like that nasty annual car registration fee or other large annual/semi-annual bill or Christmas or vacation or whatever you budget for.

So I word the PSA a little differently this time: Don't get fooled by the cleared balance - its far from your real balance. Your real balance is your cleared balance minus the uncleared checks (or failed auto-pays) minus the money you already promised you were saving for something (i.e. budget categories).