Advice needed..Offended someone accidentally?

Ok, well it all started like this..

I was having a little get together for my father's birthday (emphasis on little, only my parents, grandparents and my husband, kids and I), and the set time for arrival was 4:30pm. Well, I had some things I needed at the store before everyone arrived..I was busy all day, and didn't get to leave the house till 3:50pm...well, needless to say try as I might, I wasn't going to arrive before the first guest (my grandmother who is always early), but my husband and the kids were home to welcome guests, so I figured he'd be welcoming people in & I'd be back home a few min. after, so it wouldn't be so bad....well, 4:20pm while I am leaving the store, my Grandma calls and says how she is there, at my house....well I'm assuming that she's already inside cuz my husband is home. I tell her that I had to get some last minute personal stuff (true) and I'll be home very soon. She somehow thought I said that "WE had to pick up some personal stuff @ store". When she had pulled up to the house in the driveway, she saw how my car wasn't there & assumed that absolutely nobody was home. I had no idea she thought this, my husband's car was there. While on the phone with her I said how I would be home real soon, and she sounded quite aggravated. I didn't think at all to tell her go inside, cuzz I thought she was already inside. Well we hang up, and on my way home (driving) I get a call from my husband saying where are you? everyone is arriving. Well, I told him i will be home in like 10 min., I was on my way...he says ok and we hang up. well about 2 min from my house I drive past my Grandma going the other way, leaving my house. I get home and my husband is on the phone with her and that's when I discover she thought nobody was home...my husband did see her in the driveway, but assumed she was going to get out and come to the door as she always does and everyone else always does, but of course she isn't going to do that if she thinks nobody is home. Well, after a couple min. my husband, who is watching 2 of the kids (age 4 & 2) is wondering why she hasn't come to the door. He goes to the window to check and sees her pulling out of the driveway. So he calls her and he discovers the big problem, she thought nobody was home...and that's when I walk in, while he's on the phone with her. He mouths to me how she's mad @ me..and I am confused about things until my husband mouths that she apparently heard me say "we were out.." instead of "I was out.." So he just hands the phone over to me and she proceeds in telling me how mad at me she is, and why did my husband not go outside to greet her? And how the last minute trip to the store was just like my mother (who is know by all as "last minute sally" to the extreme, she really is always late). Well i agreed with her that the trip was ill timed, and apologized for not being home. She was mad at my husband, at me, and my parents cuz by that time it was 4:45 or sometime just before 5pm, and of course they were not there on time...so I tried to get my Grandma to turn around and com back, but to no avail. Her and my Grandpa were real hungry and left to go eat. She was in such a mood from it all, that she couldn't turn around and come back...I guess it all just upset her so much.

Now what do you think i should do? I know that my Grandma is hyper sensitive, she's known for it, but none the less, it was kind of out fault...it was all one big misunderstanding & miscommunication. I have learned from the experience to try to b more communicative & to time things more carefully. What should I do about my grandma tho? What would you do?