Not sure if this is the right place, but since a few wallet authors congregate here I though it might be the best place.
It seems every once in a while you see stories of people accidentally paying huge fees. Today I read about a man who paid a 20.14BTC fee for a 0.05 BTC transaction[1], oops. There was another recently where someone paid a fee of about 200BTC which fortunately the pool operator refunded.
It just occurs to me this kind of sad story could be averted if wallets implemented a confirmation box if the fee amount seems crazy - for example, if it's >10x what the default fee should be, or if it's greater than x% of the sending amount. "the fee seems unusually high, are you really sure you want to pay X in fees?"
I realise the exact details of this might need to be fleshed out given we want flexible fees, but it should be pretty simple to agree with what looks like an unusually large fee according to the going rate.
Drak