are carry trades not an almost guaranteed way to make money?

John M asked:


if you borrow in a low interest rate currency, such as the yen and convert it into a currency with a high interest rate, such as british pounds, is that not a guaranteed way to make money, as long as you can be fairly sure that there will not be a massive change in the exchange rate.

i know that theres no such thing as risk free profit, so i was wondering what the catch is to doing this, as i cant see it.