( saying ) you should not be too confident that sth will be successful, because sth may still go wrong
DON'T COUNT YOUR CHICKENS (BEFORE THEY ARE HATCHED)
Meaning of DON'T COUNT YOUR CHICKENS (BEFORE THEY ARE HATCHED) in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005