[lxgr]

But if they do, can't you just call your bank and ask for the ACH debit to be reversed?

I've never had to actually do this myself, but as far as I understand, there's a process for that just like for credit card chargebacks, and a legal requirement to do so within 10 days of reporting the problem to your bank.

Of course being out the money for 10 days is not pleasant; I also do wish there were ways to limit ACH payments a bit more proactively.