Miscalculation
noun, する verb
A mistake in what you calculate is a miscalculation.
ごさん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
彼女は英語がペラペラだと思ったが、とんだ誤算だった。
I was wrong in thinking that she would speak decent English.