What is with "casual" lying?

What is the deal with people who lie because they can't be bothered to think something through or because they don't know or remember something?

DH has been one such person. It took me years to figure out what was going on. He is hard of hearing so initially I thought he was embarrassed but it turns out that he does this with events that don't involve hearing. Today, he got us into some real trouble. He told me from his hospital room that I needed to fill out a form for his mom's insurance so that she can stay on Medicaid. I have been filling out this form every year for four years but for some reason, they sent it to her nursing home instead of our house. He told me that the social director had given him the form two weeks ago when he went in for a consultation about his mom. The form was due 11/7 but he said that he was told that it could be turned in as late as 12/6 which is why they didn't give it to him sooner. Mind you, MIL is in late stage Alzheimers and if she gets kicked out, I don't know what I'll do.

So I took the form in today and explained to the woman in the business office what DH had told me and she said, "You know how you repeat things that your parents said to you when you were a child? Well, my dad used to say 'You catch more flies with honey than with vinegar'". I didn't understand what she was saying as I was being very nice about it - really just explaining what I understood had happened. So I made a joke about paperwork, paid MIL's bill and left.

Everything went wrong this week. If I started, I wouldn't stop. Needless to say, I'm having stress headaches and got dizzy the other day for no apparent reason. I started to freak out about today's encounter and called DH to let him know that I was at my limit and I needed help where he could. First he got testy. Then a little nasty. I told him that I needed his support because I have a ton of stuff to do now that he's out of commission with his broken leg. Finally, he said, "Well, that's not how it happened." He told me that they had called him to come pick up the form at the beginning of November and he did. It's been sitting on his desk ever since. Then he told me to call them tomorrow and talk to the social director since it was so late and clearly stated that she would be off Medicaid if they didn't get it in time. I said, why would I talk to the social director about insurance? He said, well I could call the business office, but it would be easier for you to do it. I said, only for you. Then he said he would call and hung up on me.

I do all of the paperwork in our house and handle all of our finances, including his mom's. This is just insane. I don't get it. If they cancel her Medicaid, she gets kicked out of the nursing home. Years ago, I said to him, "If I die, this is the only piece of paper that you absolutely *have* to fill out for your mom. Otherwise, she's coming home." And now my relationship with the nursing home is seriously compromised. I am completely humiliated. It's as if DH doesn't have any idea of how to get things done.

What do you do with "casual" liars?