I can’t see anything wrong with the picture. 
Here’s a hint:
Look at the cross bar under the arm rest on the far right of the bench.
I saw that, but it still looks to me like the bench part is metal instead of wood, so not actually missing.
Look at the shadows underneath, particularly the bum of the girl on the left.
Took me about 5 seconds. But then, I was always good at the Highlights magazine game where you had to find what is wrong with this picture.
Oh GAH finally! Terrible at this. I looked, looked again, went away so I wouldn’t feel too dumb, looked again, and am so thankful to just have the answer. 
Ok wait I thought it was:
The girl on the right arm(s)
I don’t see anything wrong with that.
There’s nothing wrong with that but I can see why you thought it.
So no one else sees
The girl on the right has 2 left arm/hands?!
It’s just the way she’s sitting, as the punchline goes! What looks like an arm reaching across is her boobs. Her arm is tucked up behind her - elbow on the back of the bench.
I did NOT see that until you described it and now I’m laughing my ass off! 


(But I still don’t think that was the intention of whoever posted it.)
I “think” I see it now but,
That hand still looks like a lefty! 

I don’t know why I’m still blurring my wrong answer!? 
It’s fun for other people to guess your wrong answer 
Maybe it’s time for new glasses! 
I’m kind of glad some others are having a hard time seeing it too. I was starting to feel like the slow one
For what it’s worth, I did find my wrong answer pretty quick. Then read the blurred things and was confused 
Bench pic talk:
Their butts all seem a good 4-6 in. higher than the plane on which the seat would have been too, assuming level with the crossbar. That led me to the issue pretty quickly as a bit of a sloppy photoshop. There seems to be a shadow of the seat too, but that’s trickier to verify considering the positions of the sun and people and the other shadow components to contend with. Another fun thread idea: photoshopped WDW pics where we have to find what’s been altered.
I got it fairly quickly but I also saw this a long time ago I just couldn’t remember what was missing . I had to do some intense looking, lol.
This is my issue with the picture. Definitely sloppy work!