Fig.: to start something that will lead to greater unforeseen troubles and problems

If they can have him testifying in court, it will open a Pandora's box on bribes and sex scandals in town

Se riusciranno a farlo testimoniare in aula, si scoperchierà il vaso di Pandora sulle tangenti e sugli scandali sessuali in città

My lawyer is advising me against filing this claim, because it could open a Pandora's box of huge problems for myself and my family, and once the Pandora's box is open you can't put it back

Il mio avvocato mi sconsiglia di procedere perché dice che questa mia rivendicazione potrebbe aprire un vaso di Pandora di grossi problemi per me e per la mia famiglia e, una volta aperto, il vaso non si può richiudere

The Pandora myth is taken from Greek mythology, and the most famous version of it comes from Hesiod's Works and Days (lines 60-105).
Prometheus offended the gods by stealing the fire from heaven and giving it to mankind, which originally consisted only of men. In revenge, Zeus – the king of the gods – ordered the creation of the first woman. On Zeus' instructions, Hephaestus created her from clay, shaping her perfectly, Aphrodite gave her femininity, Athena taught her crafts and Hermes gave her a shameless mind and a deceitful nature. He also called her Pandora, which meant “all gifts”.
Pandora was presented with a box containing special gifts from the gods, but she was told never to open it. Then she was taken to Epimetheus, brother of Prometheus, to be his wife. Eventually she was overcome by her curiosity and opened the box, thus releasing sickness, death and evil into the world. Pandora tried to close the box as fast as possible, but it was too late. Only Hope was left inside.