〜（よ）うものなら

Used to say that if the speaker or someone merely did a certain thing, an exaggerated or very bad consequence would follow. The second clause is almost always negative: someone gets angry, something goes wrong, trouble follows. It is emphatic and somewhat literary/dramatic, common in written Japanese and storytelling; do not confuse it with 〜ものなら (as in 行けるものなら行きたい), which attaches to the potential form and expresses a wish about something hard to realize.

• 父に本当のことを言おうものなら、大変なことになる。

  ちちにほんとうのことをいおうものなら、たいへんなことになる。

  If I were to tell my father the truth, things would get terrible.

• この工場では、一つでもミスをしようものなら、すぐに責任を問われる。

  このこうじょうでは、ひとつでもミスをしようものなら、すぐにせきにんをとわれる。

  At this factory, if you make even one mistake, you are immediately held responsible.

• 彼女の前でその話をしようものなら、一晩中文句を聞かされるだろう。

  かのじょのまえでそのはなしをしようものなら、ひとばんじゅうもんくをきかされるだろう。

  If you brought that topic up in front of her, you'd have to listen to her complaints all night.