신선놀음에 도낏자루 썩는 줄 모른다

Etymology
lit. 'Caught up in the Daoist immortals' game [of Go], you forget that the handle of your hatchet is rotting'. From the  by Ren Fang (lived 460—508 AD), a Chinese writer:

Proverb

 * 1) time flies when you're having fun
 * 2)  If one is too focused on entertainment, one might forget something important