noun
mistake; error; blunder; failure
Refers to a mistake or failure in doing something, often implying a botched attempt or an error in execution. Slightly colloquial.
計算をやり損じて、合計が合わなかった。
I made a mistake in the calculation, and the total didn't add up.
彼は大事なプレゼンでやり損じ
He blundered during the important presentation.
失敗 is a general term for failure or mistake, while 遣り損じ emphasizes a mistake made in the process of doing something, often with a nuance of clumsiness or a slip-up.
間違い is a broad term for any kind of error or mistake, whereas 遣り損じ specifically refers to an error in execution or a failed attempt.
Compound of 遣り (the continuative form of 遣る, 'to do') and 損じ (the continuative form of 損じる, 'to fail' or 'to make a mistake'). The combination literally means 'doing-failure'.