.... and makes me feel dumb.
You are dumb.... but so is everyone else in this thread. We have all done exactly the same thing, be it memory cards, batteries, or any number of other important items. Once the initial "Doh" passes you will shake your head and laugh about it.
I once hiked up a big hill with a full pack of kit including lenses, camera, tripod, etc to shoot some landscape shots. When I got there I realised I had taken the tripod quick release plate off my camera two days before and left it in the cupboard. I beat myself with the tripod all the way back home.
Glad to hear it wasn't a paid shoot and glad the client is fine with it. Pretty soon checking your kit will become second nature. Until it does it may be helpful to produce a list of every piece of photo kit you have. On the evening before the shoot go through the list and decide if you will need that item for the next day's shoot. If so add it to the bag so it is ready in the morning.
Oh yea, if you don't already, get into the habit of putting batteries on to charge as soon as you get home from a shoot. Realising, as you are about to leave the house, that you forgot to charge a battery is another classic.