Thats the most common error, really easy to make accidently, but I wonder how the artists dont see it themselves before submitting their pictures to where ever they are submitting them. Every artist should look very closely to their pictures before deciding if its ready or not :/
I made this error once too, but when looking at the whole drawing it was quite easy to see.

And where exactly is the error? I don't see what's wrong with this picture...

Look at bottom girl's right toes.

Two left feet... again <.<