There are a number of tricks you can play on your friends in Harry Potter's world. One is the dreaded curse of 'jelly-legs'. What exactly is 'jelly-legs'? Jelly, in British English, is a sweetish fruit-flavoured substance, generally coloured, that is often eaten with ice cream. It's relatively firm and holds its shape quite well but is somewhat elastic because it contains gelatin. As all children know, jelly wobbles if it is touched. So jelly-legs as described by Rowling is a condition where the legs go all wobbly and the owner has trouble standing up. Lots of fun if you're a kid.
But the Taiwanese translator translates jelly-legs as 果醬腿 guǒ-jiàng-tuǐ, ie, 'jam legs'. Jam is completely different from jelly - it's sticky and sweet and is normally spread on bread. It's hard to imagine what jam legs might be - legs that are runny like jam? Or something like 'toe jam', which is a smelly substance that builds up between the toes if you don't wash your feet for a week?
I think not. In fact, the problem arises from a difference between British and American English. Jelly in Britain is what is known as Jell-O in America. Jelly in America is what is known as jam in Britain. (Actually, the distinction is somewhat more subtle: Jam in America is actually closer to a conserve or preserve; jelly is a thinner spread with less fruity lumps).
So what has happened is: the Taiwanese translator, failing to notice this difference in usage, translated 'jelly-legs' as 'jam-legs' (or if you prefer, translated 'jell-O legs' as 'jelly legs'). Whatever the reason, jam-legs probably isn't quite what J. K. Rowling had in mind.
The Mainland translation uses a somewhat more comprehensible term 打折腿 dǎ-shé-tuǐ, 'broken legs'. The Japanese version uses the term クラゲ脚 kurage-ashi ('jellyfish legs'). Vietnamese uses chân cà vẹo, literally 'twisted limping legs' .