蓦然回首回头一看是条狗
[mò rán huí shŏu huí tóu yī kàn shì tiáo gŏu]
This translates humorously or ironically to 'Turning back, only to find a dog there.' It suggests being disappointed when expectations do not match reality or feeling mocked by unforeseen circumstances. It also conveys a casual and perhaps cynical attitude.