一不小心

yībùxiǎoxīn
to be a little bit careless; to have a moment of inattentiveness; accidentally

CEDICT

Entry Composition

1 一 yī
  1. one
  2. single
  3. a (article)
  4. as soon as
  5. entire; whole; all; throughout
  6. "one" radical in Chinese characters (Kangxi radical 1)
  7. also pr. yāo for greater clarity when spelling out numbers digit by digit
2 不 bù
  1. no; not so
  2. (bound form) not; un-
3 小心 xiǎoxīn
  1. to be careful
  2. to take care