где тонко, там и рвётся

посл.

lit. the cloth's tears where it is thin; cf. the chain is as strong as (no stronger than) its weakest link; the thread breaks where it is weakest; the worst spoke in a cart breaks first

- Всё-то, матушка Степановна, горе, горе одно, ведь где тонко, там и рвётся. (В. Белов, Привычное дело) — 'Eh, ye can see, Stepanovna, it's just sorrow and grief, sorrow and grief, where the cloth's thin, there it tears.'