Yesterday, Yesung accidentally tweeted a photo reading 'f*ck' instead of 'luck'. The word was written in cursive and it's easy to see how Yesung, who isn't fluent, could have made the error.
He posted the photo on his Instagram as well with the tags "#f #no #l", and was also told by a friend that the letter was indeed an f instead of an l, and he'd basically posted "good f*ck today". Yesung sheepishly responded, "Oh no... If I erase it now, it's embarrassing, right?"
The tweet still exists here, so he seems like he's decided to just stick with it. Well, at least now he knows what's going on!