They [Percy’s twin younger brothers] frog-marched Percy from the room, his arms pinned to his side by his sweater.
(Harry Potter and the Sorcerer's Stone)
They pinned Percy’s ‘two’ arms. But why isn’t the ‘side’ plural form?
This appears to be an error introduced when the text was modified for the US edition. In the original UK edition it reads:
‘I – don’t – want –’ said Percy thickly, as the twins forced the jumper over his head, knocking his glasses askew.
‘And you’re not sitting with the Prefects today, either,’ said George. ‘Christmas is a time for family.’
They frog-marched Percy from the room, his arms pinned to his sides by his jumper.
Note that ‘jumper’ has been changed to ‘sweater’, to conform to US usage.