<human>: Summarize the given text in a few sentences.

So I am in the military and my family and I rarely get to go "home" and see our parents and grandparents.  So twice a year we try to make it home to see them.

Fast forward and we are at my aunts house and my grandparents are also over to visit with us.  My wife and I were looking at something out the patio window which looks into a forest.  I walked away to check on our son who was in the other room for a quick minute.  As I return I thought my wife was still looking at the window dark hair, dark pants, about 5 foot tall, and so being the perv I am I proceed to walk up and slap "her" on the ass with a playful tap.  As my hand meets I end with a grab and that was when it hit me.

I realized the ass I was grabbing was not my wife's tight young perfectly round bottom and was instead a flabby flatter rear side.  My grandmother turns around and we make eye contact, I am obviously instantly mortified and go into retard mode and blurt out, "Thats not (wifes names) ass".  You would think I said it in a surprised tone, but in my dumbfounded state I said it in a matter of fact tone.  And I just leave the room.  

I can never live it down, knowing that I basically fondled my grandmother's butt.  No matter how hard I try I can't forget the feeling "not a good one".  Luckily they all thought it was the funniest shit ever and agreed it was a reasonable mistake given the hair color and clothing resemblance.  But yeah regardless it was a pretty epic fuck up none the less.
<bot>: I slapped/grabbed my grandmothers ass in the most sensual way possible in front of half of my family.