Meaning: when impossible things happen. Meaning and uses.
Rewrite 1: when trees dance (can you imagine them in Riverdance?)
Rewrite 2: when cats drive
Rewrite 3: when horses skip rope (they can jump but can they skip?)
Rewrite 4: when penguins fly
Rewrite 5: when cars parallel-park themselves
Comment: It sounds awkward when you add many words, as in rewrites 3 and 5, so simpler statements may work better. However, I encourage more imaginative statements, more impossible combinations.
More reading about clichés
What I found when I googled “clichés”:
• Cheats, Cliches, Cartoons, Anime…: “…many ways to cut corners in creating animation.”