Yesung made an honest mistake on Twitter.
He posted, "Good luck today" along with a photo of an image that read 'luck' with a heart behind it. Or, well, what was supposed to read 'luck'. The photo actually reads 'f*ck', but is written in cursive and it's easy to see how Yesung, who isn't fluent, could have made the error.
Fans have gently pointed this out to him and some find the tweet hilarious, but at time of writing, he has not yet deleted the tweet. (Perhaps he hasn't checked his mentions yet?)
You can go see the tweet here. If the link doesn't work, he's probably deleted the tweet, but you can still check out the screenshot above of the cute (and pretty funny) mistake.